diff --git a/src/calibre/db/backend.py b/src/calibre/db/backend.py
index 4240333bda..865b65ffa9 100644
--- a/src/calibre/db/backend.py
+++ b/src/calibre/db/backend.py
@@ -973,8 +973,8 @@ class DB:
     def remove_dirty_fts(self, book_id, fmt):
         return self.fts.remove_dirty(book_id, fmt)
 
-    def queue_fts_job(self, book_id, fmt, path, fmt_size, fmt_hash):
-        return self.fts.queue_job(book_id, fmt, path, fmt_size, fmt_hash)
+    def queue_fts_job(self, book_id, fmt, path, fmt_size, fmt_hash, start_time):
+        return self.fts.queue_job(book_id, fmt, path, fmt_size, fmt_hash, start_time)
 
     def commit_fts_result(self, book_id, fmt, fmt_size, fmt_hash, text, err_msg):
         if self.fts is not None:
diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index 14a8279b3e..6f665f83b4 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -19,9 +19,9 @@ from functools import partial, wraps
 from io import DEFAULT_BUFFER_SIZE, BytesIO
 from queue import Queue
 from threading import Lock
-from time import sleep, time
+from time import monotonic, sleep, time
 
-from calibre import as_unicode, isbytestring, detect_ncpus
+from calibre import as_unicode, detect_ncpus, isbytestring
 from calibre.constants import iswindows, preferred_encoding
 from calibre.customize.ui import (
     run_plugins_on_import, run_plugins_on_postadd, run_plugins_on_postimport
@@ -437,6 +437,8 @@ class Cache:
     # FTS API {{{
     def initialize_fts(self):
         self.fts_queue_thread = None
+        self.fts_measuring_rate = None
+        self.fts_num_done_since_start = 0
         self.fts_job_queue = Queue()
         self.fts_indexing_left = self.fts_indexing_total = 0
         fts = self.backend.initialize_fts(weakref.ref(self))
@@ -445,7 +447,7 @@ class Cache:
         return fts
 
     def start_fts_pool(self):
-        from threading import Thread, Event
+        from threading import Event, Thread
         self.fts_dispatch_stop_event = Event()
         self.fts_queue_thread = Thread(name='FTSQueue', target=Cache.dispatch_fts_jobs, args=(
             self.fts_job_queue, self.fts_dispatch_stop_event, weakref.ref(self)), daemon=True)
@@ -458,20 +460,32 @@ class Cache:
     def is_fts_enabled(self):
         return self.backend.fts_enabled
 
-    @api
-    def fts_indexing_progress(self):
-        return self.fts_indexing_left, self.fts_indexing_total
+    @write_api
+    def fts_start_measuring_rate(self, measure=True):
+        self.fts_measuring_rate = monotonic() if measure else None
+        self.fts_num_done_since_start = 0
 
-    def _update_fts_indexing_numbers(self):
+    def _update_fts_indexing_numbers(self, job_time=None):
         # this is called when new formats are added and when a format is
         # indexed, but NOT when books or formats are deleted, so total may not
         # be up to date.
         nl = self.backend.fts.number_dirtied()
         nt = self.backend.get('SELECT COUNT(*) FROM main.data')[0][0] or 0
-        if (self.fts_indexing_left, self.fts_indexing_total) != (nl, nt):
+        if not nl:
+            self._fts_start_measuring_rate(measure=False)
+        if job_time is not None and self.fts_measuring_rate is not None:
+            self.fts_num_done_since_start += 1
+        if (self.fts_indexing_left, self.fts_indexing_total) != (nl, nt) or job_time is not None:
             self.fts_indexing_left = nl
             self.fts_indexing_total = nt
-            self.event_dispatcher(EventType.indexing_progress_changed, nl, nt)
+            self.event_dispatcher(EventType.indexing_progress_changed, *self._fts_indexing_progress())
+
+    @read_api
+    def fts_indexing_progress(self):
+        rate = None
+        if self.fts_measuring_rate is not None and self.fts_num_done_since_start > 4:
+            rate = self.fts_num_done_since_start / (monotonic() - self.fts_measuring_rate)
+        return self.fts_indexing_left, self.fts_indexing_total, rate
 
     @write_api
     def enable_fts(self, enabled=True, start_pool=True):
@@ -498,6 +512,7 @@ class Cache:
             self = dbref()
             if self is None:
                 return False
+            start_time = monotonic()
             with self.read_lock:
                 if not self.backend.fts_enabled:
                     return False
@@ -522,9 +537,9 @@ class Cache:
                     h.update(chunk)
                     pt.write(chunk)
             with self.write_lock:
-                queued = self.backend.queue_fts_job(book_id, fmt, pt.name, sz, h.hexdigest())
+                queued = self.backend.queue_fts_job(book_id, fmt, pt.name, sz, h.hexdigest(), start_time)
                 if not queued:  # means a dirtied book was removed from the dirty list because the text has not changed
-                    self._update_fts_indexing_numbers()
+                    self._update_fts_indexing_numbers(monotonic() - start_time)
                 return self.backend.fts_has_idle_workers
 
         def loop_while_more_available():
@@ -555,9 +570,9 @@ class Cache:
         self._update_fts_indexing_numbers()
 
     @write_api
-    def commit_fts_result(self, book_id, fmt, fmt_size, fmt_hash, text, err_msg):
+    def commit_fts_result(self, book_id, fmt, fmt_size, fmt_hash, text, err_msg, start_time):
         ans = self.backend.commit_fts_result(book_id, fmt, fmt_size, fmt_hash, text, err_msg)
-        self._update_fts_indexing_numbers()
+        self._update_fts_indexing_numbers(monotonic() - start_time)
         return ans
 
     @write_api
@@ -583,23 +598,29 @@ class Cache:
             self._queue_next_fts_job()
         return fts
 
-    @api
-    def set_fts_num_of_workers(self, num=None):
+    @write_api
+    def set_fts_num_of_workers(self, num):
         existing = self.backend.fts_num_of_workers
-        if num is not None:
+        if num != existing:
             self.backend.fts_num_of_workers = num
             if num > existing:
-                self.queue_next_fts_job()
-        return existing
+                self._queue_next_fts_job()
+            return True
+        return False
 
-    @api
+    @write_api
     def set_fts_speed(self, slow=True):
+        orig = self.fts_indexing_sleep_time
         if slow:
             self.fts_indexing_sleep_time = Cache.fts_indexing_sleep_time
-            self.set_fts_num_of_workers(1)
+            changed = self._set_fts_num_of_workers(1)
         else:
             self.fts_indexing_sleep_time = 0.1
-            self.set_fts_num_of_workers(max(1, detect_ncpus()))
+            changed = self._set_fts_num_of_workers(max(1, detect_ncpus()))
+        changed = changed or orig != self.fts_indexing_sleep_time
+        if changed and self.fts_measuring_rate is not None:
+            self._fts_start_measuring_rate()
+        return changed
 
     @read_api
     def fts_search(
@@ -967,7 +988,7 @@ class Cache:
                     return
             ret = pt.name
         elif as_pixmap or as_image:
-            from qt.core import QPixmap, QImage
+            from qt.core import QImage, QPixmap
             ret = QImage() if as_image else QPixmap()
             with self.safe_read_lock:
                 path = self._format_abspath(book_id, '__COVER_INTERNAL__')
diff --git a/src/calibre/db/fts/connect.py b/src/calibre/db/fts/connect.py
index 6e31fb30a3..90faebcc86 100644
--- a/src/calibre/db/fts/connect.py
+++ b/src/calibre/db/fts/connect.py
@@ -133,14 +133,14 @@ class FTS:
                 break
         self.add_text(book_id, fmt, text, text_hash, fmt_size, fmt_hash, err_msg)
 
-    def queue_job(self, book_id, fmt, path, fmt_size, fmt_hash):
+    def queue_job(self, book_id, fmt, path, fmt_size, fmt_hash, start_time):
         conn = self.get_connection()
         fmt = fmt.upper()
         for x in conn.get('SELECT id FROM fts_db.books_text WHERE book=? AND format=? AND format_size=? AND format_hash=?', (
                 book_id, fmt, fmt_size, fmt_hash)):
             break
         else:
-            self.pool.add_job(book_id, fmt, path, fmt_size, fmt_hash)
+            self.pool.add_job(book_id, fmt, path, fmt_size, fmt_hash, start_time)
             conn.execute('UPDATE fts_db.dirtied_formats SET in_progress=TRUE WHERE book=? AND format=?', (book_id, fmt))
             return True
         self.remove_dirty(book_id, fmt)
diff --git a/src/calibre/db/fts/pool.py b/src/calibre/db/fts/pool.py
index 995a55c265..34c3477c19 100644
--- a/src/calibre/db/fts/pool.py
+++ b/src/calibre/db/fts/pool.py
@@ -21,12 +21,13 @@ quit = object()
 
 class Job:
 
-    def __init__(self, book_id, fmt, path, fmt_size, fmt_hash):
+    def __init__(self, book_id, fmt, path, fmt_size, fmt_hash, start_time):
         self.book_id = book_id
         self.fmt = fmt
         self.fmt_size = fmt_size
         self.fmt_hash = fmt_hash
         self.path = path
+        self.start_time = start_time
 
 
 class Result:
@@ -37,6 +38,7 @@ class Result:
         self.fmt_size = job.fmt_size
         self.fmt_hash = job.fmt_hash
         self.ok = not bool(err_msg)
+        self.start_time = job.start_time
         if self.ok:
             with open(job.path + '.txt', 'rb') as src:
                 try:
@@ -177,9 +179,9 @@ class Pool:
         self.initialize()
         self.supervise_queue.put(check_for_work)
 
-    def add_job(self, book_id, fmt, path, fmt_size, fmt_hash):
+    def add_job(self, book_id, fmt, path, fmt_size, fmt_hash, start_time):
         self.initialize()
-        job = Job(book_id, fmt, path, fmt_size, fmt_hash)
+        job = Job(book_id, fmt, path, fmt_size, fmt_hash, start_time)
         self.jobs_queue.put(job)
 
     def commit_result(self, result):
@@ -192,7 +194,7 @@ class Pool:
             text = ''
         db = self.dbref()
         if db is not None:
-            db.commit_fts_result(result.book_id, result.fmt, result.fmt_size, result.fmt_hash, text, err_msg)
+            db.commit_fts_result(result.book_id, result.fmt, result.fmt_size, result.fmt_hash, text, err_msg, result.start_time)
 
     def shutdown(self):
         if self.initialized.is_set():
diff --git a/src/calibre/db/utils.py b/src/calibre/db/utils.py
index a96856530f..5463c17047 100644
--- a/src/calibre/db/utils.py
+++ b/src/calibre/db/utils.py
@@ -4,7 +4,7 @@
 __license__ = 'GPL v3'
 __copyright__ = '2013, Kovid Goyal <kovid at kovidgoyal.net>'
 
-import os, errno, sys, re, time
+import os, errno, sys, re
 from locale import localeconv
 from collections import OrderedDict, namedtuple
 from polyglot.builtins import iteritems, itervalues, string_or_bytes
@@ -458,24 +458,15 @@ class IndexingProgress:
         self.reset()
 
     def __repr__(self):
-        return f'IndexingProgress(left={self.left}, total={self.total})'
-
-    def clear_rate_information(self):
-        from collections import deque
-        self.done_events = deque()
+        return f'IndexingProgress(left={self.left}, total={self.total}, rate={self.indexing_rate})'
 
     def reset(self):
         self.left = self.total = -1
-        self.clear_rate_information()
+        self.indexing_rate = None
 
-    def update(self, left, total):
-        changed = (left, total) != (self.left, self.total)
-        if changed:
-            done_num = self.left - left
-            if done_num > 0 and self.left > -1:  # initial event will have self.left == -1
-                self.done_events.append((done_num, time.monotonic()))
-                if len(self.done_events) > 50:
-                    self.done_events.popleft()
+    def update(self, left, total, indexing_rate):
+        changed = (left, total, indexing_rate) != (self.left, self.total, self.indexing_rate)
+        self.indexing_rate = indexing_rate
         self.left, self.total = left, total
         return changed
 
@@ -493,14 +484,10 @@ class IndexingProgress:
             return _('calculating time left')
         if self.left < 2:
             return _('almost done')
-        if len(self.done_events) < 5:
+        if self.indexing_rate is None:
             return _('calculating time left')
         try:
-            start_time = self.done_events[0][1]
-            end_time = self.done_events[-1][1]
-            num_done = sum(x[0] for x in self.done_events) - self.done_events[0][0]
-            rate = num_done / max(0.1, end_time - start_time)
-            seconds_left = self.left / rate
+            seconds_left = self.left / self.indexing_rate
             return _('~{} left').format(human_readable_interval(seconds_left))
         except Exception:
             return _('calculating time left')
diff --git a/src/calibre/gui2/fts/scan.py b/src/calibre/gui2/fts/scan.py
index c53a504376..0ccfeb3d5f 100644
--- a/src/calibre/gui2/fts/scan.py
+++ b/src/calibre/gui2/fts/scan.py
@@ -70,7 +70,6 @@ class ScanProgress(QWidget):
     def change_speed(self):
         db = get_db()
         db.set_fts_speed(slow=not self.fast_button.isChecked())
-        self.indexing_progress.clear_rate_information()
 
     def update(self, complete, left, total):
         if complete:
@@ -130,14 +129,15 @@ class ScanStatus(QWidget):
 
     def gui_update_event(self, db, event_type, event_data):
         if event_type is EventType.indexing_progress_changed:
-            self.update_stats()
+            self.update_stats(event_data)
 
     def __call__(self, event_type, library_id, event_data):
         if event_type is EventType.indexing_progress_changed:
-            self.update_stats()
+            self.update_stats(event_data)
 
-    def update_stats(self):
-        changed = self.indexing_progress.update(*self.db.fts_indexing_progress())
+    def update_stats(self, event_data=None):
+        event_data = event_data or self.db.fts_indexing_progress()
+        changed = self.indexing_progress.update(*event_data)
         if changed:
             self.indexing_progress_changed.emit(self.indexing_progress.complete, self.indexing_progress.left, self.indexing_progress.total)
 
@@ -194,12 +194,12 @@ class ScanStatus(QWidget):
         self.apply_fts_state()
 
     def startup(self):
-        self.indexing_progress.clear_rate_information()
+        self.db.fts_start_measuring_rate(measure=True)
 
     def shutdown(self):
         self.scan_progress.slow_button.setChecked(True)
         self.reset_indexing_state_for_current_db()
-        self.indexing_progress.clear_rate_information()
+        self.db.fts_start_measuring_rate(measure=False)
 
 
 if __name__ == '__main__':