Exiting Writer's Print Preview mode is slow in long documents

[Max Barry]

(Cross-posting from https://bugs.documentfoundation.org/show_bug.cgi?id=129900 as problem not reproducible on Windows.)

Compared to LibreOffice 5, exiting Print Preview in long documents on LO6 is extremely slow.

With a 1,500 page document, for example, exiting Print Preview is instant in LO5, but takes 5-6 seconds (!) in LO6.

To reproduce:

1. In LibreOffice 6 Writer, fill a document with text so that it comprises at least a few hundred pages.

2. Enter Print Preview (e.g. File -> Print Preview).

3. Observe it takes a second or two (based on document length) to display the Print Preview screen. This is similar to LibreOffice 5.

4. Press Escape to exit Print Preview mode.

5. Observe a significant delay (approx. 5-6 seconds for a 1,500 page document) with CPU usage spiking before returning to normal view. There is no such delay in LibreOffice 5.3.0.3 Writer.

I tend to flip back and forth between Normal View and Print Preview when editing a document, and this new delay makes this very awkward.

Ubuntu 19.10

Version: 6.3.4.2
Build ID: 1:6.3.4-0ubuntu0.19.10.1
CPU threads: 4; OS: Linux 5.3; UI render: default; VCL: gtk3;
Locale: en-AU (en_AU.UTF-8); UI-Language: en-US
Calc: threaded

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: libreoffice-writer 1:6.3.4-0ubuntu0.19.10.1
ProcVersionSignature: Ubuntu 5.3.0-26.28-generic 5.3.13
Uname: Linux 5.3.0-26-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu8.2
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Jan 10 10:53:20 2020
InstallationDate: Installed on 2013-04-11 (2464 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Alpha amd64 (20130402.1)
SourcePackage: libreoffice
UpgradeStatus: Upgraded to eoan on 2019-11-07 (63 days ago)
modified.conffile..etc.default.apport:
 # set this to 0 to disable apport, or to 1 to enable it
 # you can temporarily override this with
 # sudo service apport start force_start=1
 enabled=0
mtime.conffile..etc.default.apport: 2015-03-26T08:56:12.843910