Yeah, I have the same problem with MIUI 12 (20.4.30), I haven't tried the new 20.5.7 though.
I have the best experience with MIUI (11.1.18), anything newer than that is more or less laggy.
TBH, Chinese ROM is the most stable version IMO, but can't run Google play and soooo much more Ads are the deal-breaker.