diff --git a/src/calibre/customize/conversion.py b/src/calibre/customize/conversion.py
index e98f34273f..72c067747d 100644
--- a/src/calibre/customize/conversion.py
+++ b/src/calibre/customize/conversion.py
@@ -112,6 +112,10 @@ class InputFormatPlugin(Plugin):
     #: convenience method, :meth:`get_image_collection`.
     is_image_collection = False
 
+    #: Number of CPU cores used by this plugin
+    #: A value of -1 means that it uses all available cores
+    core_usage = 1
+
     #: If set to True, the input plugin will perform special processing
     #: to make its output suitable for viewing
     for_viewer = False
diff --git a/src/calibre/ebooks/comic/input.py b/src/calibre/ebooks/comic/input.py
index bfc6ff30ca..443a302999 100755
--- a/src/calibre/ebooks/comic/input.py
+++ b/src/calibre/ebooks/comic/input.py
@@ -247,6 +247,7 @@ class ComicInput(InputFormatPlugin):
     description = 'Optimize comic files (.cbz, .cbr, .cbc) for viewing on portable devices'
     file_types  = set(['cbz', 'cbr', 'cbc'])
     is_image_collection = True
+    core_usage = -1
 
     options = set([
         OptionRecommendation(name='colors', recommended_value=256,
diff --git a/src/calibre/gui2/actions/convert.py b/src/calibre/gui2/actions/convert.py
index 0641cc6a97..ef8a8688c5 100644
--- a/src/calibre/gui2/actions/convert.py
+++ b/src/calibre/gui2/actions/convert.py
@@ -14,6 +14,7 @@ from calibre.gui2 import error_dialog, Dispatcher
 from calibre.gui2.tools import convert_single_ebook, convert_bulk_ebook
 from calibre.utils.config import prefs
 from calibre.gui2.actions import InterfaceAction
+from calibre.customize.ui import plugin_for_input_format
 
 class ConvertAction(InterfaceAction):
 
@@ -115,9 +116,19 @@ class ConvertAction(InterfaceAction):
     def queue_convert_jobs(self, jobs, changed, bad, rows, previous,
             converted_func, extra_job_args=[]):
         for func, args, desc, fmt, id, temp_files in jobs:
+            input_file = args[0]
+            input_fmt = os.path.splitext(input_file)[1]
+            core_usage = 1
+            if input_fmt:
+                input_fmt = input_fmt[1:]
+                plugin = plugin_for_input_format(input_fmt)
+                if plugin is not None:
+                    core_usage = plugin.core_usage
+
             if id not in bad:
                 job = self.gui.job_manager.run_job(Dispatcher(converted_func),
-                                            func, args=args, description=desc)
+                                            func, args=args, description=desc,
+                                            core_usage=core_usage)
                 args = [temp_files, fmt, id]+extra_job_args
                 self.conversion_jobs[job] = tuple(args)
 
diff --git a/src/calibre/gui2/jobs.py b/src/calibre/gui2/jobs.py
index d512409d25..813a9bda45 100644
--- a/src/calibre/gui2/jobs.py
+++ b/src/calibre/gui2/jobs.py
@@ -198,8 +198,9 @@ class JobManager(QAbstractTableModel):
         return False
 
     def run_job(self, done, name, args=[], kwargs={},
-                           description=''):
+                           description='', core_usage=1):
         job = ParallelJob(name, description, done, args=args, kwargs=kwargs)
+        job.core_usage = core_usage
         self.add_job(job)
         self.server.add_job(job)
         return job
diff --git a/src/calibre/utils/ipc/job.py b/src/calibre/utils/ipc/job.py
index a6c39ffc6b..91db333791 100644
--- a/src/calibre/utils/ipc/job.py
+++ b/src/calibre/utils/ipc/job.py
@@ -42,6 +42,7 @@ class BaseJob(object):
         self._message      = None
         self._status_text  = _('Waiting...')
         self._done_called  = False
+        self.core_usage    = 1
 
     def update(self, consume_notifications=True):
         if self.duration is not None: