[okular] [Bug 461353] New: Very slow panning with high zoom (espoecially on small monitors)

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https://bugs.kde.org/show_bug.cgi?id=461353

            Bug ID: 461353
           Summary: Very slow panning with high zoom (espoecially on small
                    monitors)
    Classification: Applications
           Product: okular
           Version: 22.04.3
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: PDF backend
          Assignee: okular-devel at kde.org
          Reporter: tomashnyk at gmail.com
  Target Milestone: ---

Created attachment 153435
  --> https://bugs.kde.org/attachment.cgi?id=153435&action=edit
Video fo the slowness

SUMMARY
When I zoom the map a lot, moving around is very slow (I would say mostly
unusable).

STEPS TO REPRODUCE
1. Download
https://www.parconazionaledelvesuvio.it/wp-content/uploads/2020/05/Parco-Naionale-del-Vesuvio-Guida-ai-Sentieri-del-Parco-2020-05.pdf 
(there is a size limit to bug reports of 4 MB, the file has 6.7 MB, can it be
somehow uploaded so it does not get lost?)
2. On the first page, zoom to at least 200 %
3. Press right-mouse-button and drag the map to pan it around.

OBSERVED RESULT
It is quite slow. When I pan to an area far from the original display, white
screen is displayed for quite some time before ti is displayed. When I go back
to the original area, it is not cached, instead I see again white area that
takes seconds to render. See videa attached

EXPECTED RESULT
I would expect it to all be fast and smooth. When I open the PDF with my
browser (Vivaldi) it is fast, so it can be fast. My machine is quite fast
(Thinkpad X1 Extreme gen 2, 8-core CPU etc.) As can be see in the widget above
the window, just one core is 100 % used, the others just sit there. I turned on
the greedy setting in PErformance tab of the settings. The second page of the
PDF is notably faster.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  Kubuntu 22.04
(available in About System)
KDE Plasma Version:  5.25.5
KDE Frameworks Version: 5.98.0
Qt Version:  5.15.3

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