diff --git a/setup/installer/windows/freeze.py b/setup/installer/windows/freeze.py
index 08aa91fe10..b983d09e73 100644
--- a/setup/installer/windows/freeze.py
+++ b/setup/installer/windows/freeze.py
@@ -69,6 +69,7 @@ class Win32Freeze(Command, WixMixIn):
 
     def run(self, opts):
         self.SW = SW
+        self.portable_uncompressed_size = 0
         self.opts = opts
         self.src_root = self.d(self.SRC)
         self.base = self.j(self.d(self.SRC), 'build', 'winfrozen')
@@ -407,7 +408,7 @@ class Win32Freeze(Command, WixMixIn):
 
     def build_portable_installer(self):
         zf = self.a(self.j('dist', 'calibre-portable-%s.zip.lz'%VERSION))
-        usz = os.path.getsize(zf)
+        usz = self.portable_uncompressed_size or os.path.getsize(zf)
         def cc(src, obj):
             cflags  = '/c /EHsc /MT /W4 /Ox /nologo /D_UNICODE /DUNICODE /DPSAPI_VERSION=1'.split()
             cflags.append(r'/I%s\include'%LZMA)
@@ -504,6 +505,7 @@ class Win32Freeze(Command, WixMixIn):
         with zipfile.ZipFile(name, 'w', zipfile.ZIP_STORED) as zf:
             self.add_dir_to_zip(zf, base, 'Calibre Portable')
 
+        self.portable_uncompressed_size = os.path.getsize(name)
         subprocess.check_call([LZMA + r'\bin\elzma.exe', '-9', '--lzip', name])
 
     def sign_installers(self):
diff --git a/setup/installer/windows/portable-installer.cpp b/setup/installer/windows/portable-installer.cpp
index 4929fb2312..af8281470f 100644
--- a/setup/installer/windows/portable-installer.cpp
+++ b/setup/installer/windows/portable-installer.cpp
@@ -188,7 +188,6 @@ output_callback(void *ctx, const void *buf, size_t size)
             return 0;
         }
         written = SetFilePointer(ds->out, 0, NULL, FILE_CURRENT);
-        FlushFileBuffers(ds->out); // Ensure progress is accurate, otherwise there is a long pause after the decompressing phase while the OS flushes to a slow flash drive
         ds->pd->SetProgress(written, UNCOMPRESSED_SIZE);
     }