Hi!
Thanks for your feedback.
The menu slowness is caused by an unoptimized backend - it's an issue that we're actively working on. The fix will come in Mesa v2 - a huge updated that will bring a ton of new features and fixes. Unfortunately, there's no ETA for it in order not to rush anything. All we know is it will be released in 2025.
Since Mesa is an inventory changer and not a skinchanger, mid-game skin changing doesn't work as it does in skinchangers. For maximum consistency, set your skins before the match, then join it and everything should be fine (during the warmup the issue sometimes persists, but not during the match).
Sorry for your unpleasant experience. Tune in for Mesa v2!
Have a great day!