I would try turning off Asynchronous and Interleaved Reprojection in the SteamVR settings. For some reason where these used to improve things for me, they are making lots of things worse at the moment, and things run better with them turned off. Having a Vega 64 you should not need these turned on.
Also, under the video setting click Manual Override and set it to 100%. Lots of people are having trouble with the automatic supersampling option setting the value too high and making things run badly. You can adjust it higher if that fixes the problem.