Have only had this phone for a day, but noticed the same thing.
I went to developer options and turned on 2D acceleration.
for the last two hours, it has been smooth. I also reduced the transition animations to .5x.
Just started but the screen is not smooth, it's jittery. Scrolling is just smooth. I uninstalled a couple of game app thinking that was it but it wasn't
Any thoughts on what it could be our how I can see the memory allocation