Thx, the change to LAV seems to have made it a little bit less laggy. Still only 1920p works without any lag. I have a screenshot and also wrote up some more numbers. Maybe it will give some clue to why it lags.
Local files in the SLR software:
low quality 20 % CPU, 50 % GPU (no lag)
5k 40 % CPU, 80 % GPU (just some lag)
7k 30 % CPU, 60 % GPU (massive lags, not watchable at all)
It's strange for me that it uses less CPU/GPU with the 7k video. Also the 5k video doesn't use all CPU/GPU power, so why laggy?
Online videos on SRL:
1920p 20 % CPU, 40 % GPU (no lag)
2160p 60 % CPU, 50 % GPU (just some lag)
2700p 15 % CPU, 30 % GPU (massive lags, not watchable at all)
When I have higher quality and it lags, the image can get like this pretty often as well, and even the sound can be laggy then.
https://i.imgur.com/dsaqhcm.jpg