[konsole] [Bug 468259] New: High CPU usage during intensive scrolling

[Yaroslav]

https://bugs.kde.org/show_bug.cgi?id=468259

            Bug ID: 468259
           Summary: High CPU usage during intensive scrolling
    Classification: Applications
           Product: konsole
           Version: 22.12.3
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: major
          Priority: NOR
         Component: general
          Assignee: konsole-devel at kde.org
          Reporter: yaroslav.rastrigin at gmail.com
  Target Milestone: ---

SUMMARY
***
scrolling a lot of text incurs large performance overhead. konsole is consuming
up to 65% of a single core if I'm doing, f.e. , `tail` -f of a quickly updating
log file
***


STEPS TO REPRODUCE
1. induce scrolling . easiest way is to run 'while [ 1 ] ; do echo
'testtesttesttesttest' ; done 
2. open `top` or run another performance monitor 

OBSERVED RESULT
high cpu usage, over 50% of a single core

EXPECTED RESULT
negligible cpu usage, as with other terminal emulation software

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: kde-frameworks/plasma-5.104.0:5/5.104
KDE Frameworks Version: 5.104
Qt Version: 5.15.8

ADDITIONAL INFORMATION
I have tested this with both Gentoo packages built from source by myself  , and
precompiled Arch linux packages. 
During investigation I've compiled konsole and QtGui with debugging symbols,
and ran `perf record` + `perf report` dwhile reproducing the issue. The culprit
seems to be incorrect or excessive usage of alphargbblend_argb32
     37.13%     0.00%  konsole         libQt5Gui.so.5.15.8             [.]
QtSharedPointer::ExternalRefCountWithCustomDeleter<QColorTrcLut,
QtSharedPointer::NormalDeleter>::deleter
            |
            ---QtSharedPointer::ExternalRefCountWithCustomDeleter<QColorTrcLut,
QtSharedPointer::NormalDeleter>::deleter
               0x5645ea71a9c0
               |          
               |--31.87%--alphargbblend_argb32
               |          
                --5.26%--qt_alphargbblit_argb32

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