Hello there, same issue here. I had a posting about that some time ago Strg+Z takes ages - Bug - Shotcut Forum
Daniel taught me to separate big projects into smaller projects. Thanks for that @daniel47 . I normally separate 30-minute-episodes in around 7 to 10 sub-projects. In the end they are combined in a master project file (yes, shotcut can open an .mlt in a .mlt in a.mlt).
To be honest, since 26.04 i do - sorry - i maybe - do have the feeling, that Undo became less stable. There is no actual evidence. I just noted twice, that after some minutes CPU-melt of undoing my project was rubbish. Like 30% of clips were at the wrong place, without having the chance of undoing again. Best practice is to kill Shotcut and hope for autosaved progress. I recently train myself to ctrl+alt+s backup before Undo.
Still, Shotcut is incredibly awesome. As @alex007 said: Memory Leaks are something to deal with - so i do.