Which update did you complete on the phone ?
My N20 Ultra 5G Snapdragon is on …
Samsung One Ui 3.1
Security Patch 1 June 2021.
I'm not experiencing the issues your phone is but appreciate phone's are set up different preferences and apps and have different soc in them.
Can you see what happens while your phone is in Safe Mode please.
Put the phone in Safe Mode which disables your downloaded app's but let's your stock app's keep working. If the issues stop then this highlights a downloaded app as the possible issue. Come out of Safe Mode and start deleting your most recent downloaded app's one at a time until it stops. https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/
If it happens in Safe Mode and when you've cleared your System Cache Files … Clear your phone's System Cache Files by turning your phone Off and then from a set series of button presses go into the phone's System Files and clear the cache. No information is lost this way. You may need to insert your headphones or hook upto a computer with your data cable to get to this menu on the phone. https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ as suggested by @Pugs1957 then maybe send some feedback to Samsung via the GetHelp option in your Samsung Members App.
A Samsung Experience Store or Samsung Service Centre can take a look at your phone too.
I wish you all the best with this.
Current Phone > Samsung Z Fold³ 5G.
@pjsw: Are you experiencing the issue in the UI, or any specific apps? Have you tried heading to Settings > Display > Motion Smoothness, and changing this to Adaptive to see if you notice a difference?