What do you mean by laggy? If it's a new phone, and you're still setting things up, it's normal for it to work harder, get warmer, or even feel slower at times, but once it all settles, it should be fine. Also, not every S21 Ultra is the same. Snapdragon or Exynos?
so its not stuttering, but slow animations and not smooth?
try enable developer's option ( settings - about - software info - tap build number 7 times)
go back 2 times,
developers option -> window animation scale off, transition animation scale off, animator duration scale off