[okular] [Bug 460681] New: Laggy scrolling on high DPI display - profiling indicates excessive copying in PagePainter::paintCroppedPageOnPainter

[maximumsomething]

https://bugs.kde.org/show_bug.cgi?id=460681

            Bug ID: 460681
           Summary: Laggy scrolling on high DPI display - profiling
                    indicates excessive copying in
                    PagePainter::paintCroppedPageOnPainter
    Classification: Applications
           Product: okular
           Version: unspecified
          Platform: Debian testing
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: maximumsomething at gmail.com
  Target Milestone: ---

SUMMARY
On my computer - a laptop with a 4k display set to 250% scaling - Okular
scrolls through PDFs at ~30 FPS and consumes 100% CPU while doing so. I did
some profiling, and the main thread was spending 80% of its time on
PagePainter::paintCroppedPageOnPainter. 

Looking at the source code, it's doing a few things very inefficiently:
(1) It asks the input page for a QPixmap, then dereferences it and calls
QPixmap::setDevicePixelRatio, which creates a deep copy of the pixmap;
(2) Whenever it wants to draw a cropped scaled rectangle of the pixmap, it
first creates a scaled copy of the pixmap then draws it rather than drawing it
directly.

(1) is specifically related to having a non-1 scale factor, while (2) is I
suspect just exacerbated by the high pixel count of the display.

I was going to go ahead and fix these things myself, but I was wondering if
anyone familiar with the code knows why it was written this way/any gotchas
that might apply if the deep copies are not made.

STEPS TO REPRODUCE
1. Open any PDF on a display with a high scale factor.
2. Scroll around.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma:
KDE Plasma Version:  5.25.5
KDE Frameworks Version: 5.98.0
Qt Version: 5.15.4
Graphics Platform: X11

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