As many others I've had some extreme problems with lag the last month, including sounds playing several seconds late or not at all. Animations not showing and mount appearing several seconds after mounting up.

I discovered, by accident, that for me this was caused by having several half finished quests from the Night Market in my journal. Yesterday I decided to abandon them all as I'm not planning to do any more of that content and then all my lag problems was gone. Today I've really enjoyed the game again playing with no lag.

I hope this will help others with the same problem.