There is often a noticeable lag when using the back gesture from both the left and right of the screen. You have to wait, as the back action does eventually happen. The same goes for changing the volume. You press the button and it takes a while for the overlay to appear and for the volume to actually change. Searching the forums I can see this issue raised quite a lot, but not filed as a bug. This happens on both the Mi Mix 4 and the Mi 11 Ultra.