I just upgraded to the newest anki version (2.1.42). Now I have lag while I write a card, the letters come a little late after I press a key on the keyboard. I have tried to delete all add-ons but the problem doesn’t go away.
Do you have any suggestions on what I can try next? Anki works but it just lagging while I write a new card.
Really need platform info, OS, version, anything odd with your setup. It’s like someone going to a a doctor and telling that it hurts but no other info is provided.
I kinda know that much more information is provided. 
I just really didn’t know where to start and I hoped someone else has had the same situation…
I have Windows 10 Home, 64-bit version. Anything hasn’t changed in my setup: before the upgrade, anki worked normally. I also tried to delete anki from my laptop and download it again but it didn’t change anything either.
I can reassure you, you are not alone. I think I know what you are talking about (Windows 10).
A few days ago, I switched my video driver from OpenGL to ANGLE, which seemed to fix a RAM usage problem I and other Windows users had:
The switch helped in some ways, but I still get inexplicable performance drops from time to time, especially when editing card templates. Loading the preview for a card takes a few seconds now, whereas before it was much more snappy, almost instantaneous.
I noticed the input lag you’re talking about, too. Whenever this happens, I restart Anki (old habit, used to it from the previous RAM issue anyway).
The problem is kind of hard to pinpoint down, since I have a large add-on collection. If I find a way to replicate the problem reliably without add-ons, I’ll post it here.
[got a hellweek at uni right now, my testing will have to wait until next weekend]