Too lazy to actually file a bug report. Selection area lag, which doesn’t seem to be the problem of my hardware, drivers or workload, but specifically some unnecesery operations that it performs during the movement of the rectangular area. It never happens on windows no matter how many items you select, the rectangular area always follows the cursor perfectly.
As luck would have it, someone else did file a report - searching for “selection highlight lag” on bugs.kde.org shows that this has already been filed: 388808 – RubberBand has repeated large texture uploads, causing visual lag
For what it’s worth…developers aren’t watching this forum waiting to turn free-text posts into actionable bug reports. While this particular issue has been out there for a while, for issues to ever be resolved, they need to be in a place and in a form that developers can take action on. For user-reported bugs, that is the KDE Bugzilla linked above.
If being “too lazy” is a joke, then…fair enough. If it’s legit - asking someone/multiple people to spend likely hours of volunteer time addressing your problem feels unfair if you are unwilling to spend five minutes putting it in the right format.