I don't notice any lag, everything is still smooth at 120fps. Maybe in some games? I don't play games so I don't know.
However, it uses a bit more battery, so be careful. If you have a rooted phone, you can have a script which switches between 1080p and 1644p. If you have termux (+ widgets) and sudo installed, you can even put it as a button on the power menu ! (Quick 4k toogle)
Note : I'm not an experienced user, my script may be really unoptimized or ugly, but it works!
#!/bin/bash if [ "$(sudo wm size | grep Override | cut -d "x" -f 2)" == "3840" ] then sudo wm size reset sudo wm density reset else sudo wm size 1644x3840 sudo wm density 630 fi