I'm just annoyed by the regions issues, you'll get pretty biased results depending in what region you select.
If you try to search for something specific to a region with other selected you'll find sometime empty results, which shows you won't get relevant results about a search if you don't properly select the region.
Probably this is more obvious with non technical searches, for example my default region is canada-en and if I try "instituto nacional electoral" I only get a wiki page, an international site and some other random sites with no news, only when I change the region I get the official page ine.mx and news. For me this means kagi hides results from other regions instead of just boosting the selected region's ones.
There's a rumor going that some sites slow their performance on non-chromium browsers, not sure if even on those like opera or brave which are also chromium but with I think they make more customizations.
Other thing could be how much you use each browser, how many tabs you have open when you tried them? How many extensions? What other programs did you have open?