For SMS that is more than 1500, I suggest using 3rd party SMS app such as GOSMS. I have 4800+ SMS on 1 contact (my GF, obviously :P ) and GOSMS works just fine. No stutter no lag whatsoever.
MIUIandroid 2.1.20 already released. but no changelog. weird. btw can anyone confirm last week Signed_Overscroll update is compatible with this latest miuiandroid 2.1.20? or do i need to wait?