I've asked this to multiple people in-game as well as my guild Discord and dozens of players are experiencing the same issue as me, namely fluctuating FPS (sometimes even dropping below 10, even in strongholds) which also includes delayed ability activation & basic combat action, since 6.3.
I have an RTX 3080, 16GB RAM, 1TB SSD and a 4K monitor so my rig is most definitely not the problem. I dont know if the devs have already been alerted to this, but since 6.3 the game is virtually unplayable at times.