Don't know what you did... no issue on my Mi 9T.
Used custom TWRP version? Used DFE patch before? Installed my "Universal" conversion patch (K20 Pro to Mi 9T Pro) before?
I have mi 9t and for me lag's mucy less than it was on miui 11.I do feel a lot of lag still while opening apps exiting app and scrolling . And memory usage is still high
Try clearing the Cache of the updater app. It`s just a guess, but this should fix it.