Performance issue when moving clips in or among video tracks

Shotcut - Windows 10 - presumably all versions

Steps to reproduce:
1.) Create a project with several video tracks and so many clips that moving one doesn't seem "instantaneous" - even on a very fast machine this is not more than a few hundred (the project from issue #1029 may serve as an example)
2.) Drag an arbitrary clip from the timeline within its track and WITHOUT ever leaving said track - this is your normal operation speed for reference
3.) Drag the same clip but to another track - operation takes around 100 times longer (much longer than select, Ctrl+C, position playhead, Ctrl+B, Del of the original clip would take), at least several seconds
4.) Surprisingly this also happens if you just briefly leave the orignal track while dragging, even when releasing the clip on its original track but at a different position - the operation takes a very long time

Start by reproducing the timeline drag steps in Shotcut, using a project with several tracks and many clips; issue #1029 is mentioned as a possible example. Compare moving within one track with briefly crossing into another, and consider the issue resolved when cross-track or cross-track-boundary moves no longer incur the reported large delay.