I'm afraid it's almost impossible to do anything about this without steps to reproduce. If you can find some action that you can repeat over and over and observe a performance degradation in the editor, then an issue could be filed based on that.
It could potentially be related to undo, but it's nothing more than a wild guess at this point.
If the editor is being slow a performance profile in dev tools can sometimes, but not always, offer a vague clue that might help narrow down the search a bit, but may not offer much more than that.
You could also try closing all the editor windows and tabs (e.g. project bar, layout views, event sheet views) and opening them one by one, and see if it's slow only when certain ones are open. Again that might offer a vague clue, but little more than that. As ever steps to reproduce are the gold standard.