I know there are several posts for similar issues but they have no resolution and lack information.
My issue is that when typing, sometimes the default keyboard will start to work intermittently and miss 1-4 keystrokes at a time. This issue is not touch screen related. all keys are affected. I have reset my keyboard settings, cleared the cache and deleted saved data. That has no affect. The issue is not app specific - sms, browser, snapchat, notes - all have the same issue. This happens in both portrait and landscape. I have tried 3rd party keyboards with no success. I have even gone as far as to factory reset my phone. starting completely fresh - with none of my data or apps - same thing. its utterly infuriating. The ONLY thing that somewhat works is minimizing the keyboard and then opening it back up, but this 'fix' only lasts for about 5 seconds or so. Sometimes I can use my phone for more than an hour without encountering this, but it still occurs multiple times every day and when it does, whatever your doing comes to a screeching halt. What would have taken 2 minutes to type now takes 6 or 7 as you fight with the keyboard to record your keystrokes and go back to edit what youve typed. This is clearly an issue with the note 20's as there are multiple posts about this but its not widespread and samsung refuses to replace the affected phones
Just as a further process of elimination 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.
From what you've already carried out yourself it sounds like a visit to a Samsung Experience Store or Samsung Service Centre is the next step @Brocyon
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
Definitely happening on my phone too, especially with gboard. You need to.do a lot of typing like in a doc, then memory.seems.to.run out and lag lag lag