I recently changed my graphics card from a GeForce 970 to a Vega64, and now I get very low FPS (around five-ish) with the DX11 client. Frame rates drop as soon as I enter character selection, the settings menu on the main screen before that is still fluid.
DX9 client is unaffected, everything runs fine there.
If anyone got the game running fine with a Vega and the DX11 client: I’d be grateful if you could post your settings 
EDIT: Resolved, see below.
I don’t know if that will help you, but there is that old issue, that on certain configurations setting Effects Detail to 4 (max) will cause huge FPS drops. Go to Advanced Settings and try setting Effects Detail to 3.8.
Even if that isn’t the cause of your problems, maybe it will add enough FPS to make playing possible, until you get a proper solution.
Thanks @Lubek47, already tried that, didn’t help 
Nothing of the in-game settings (including SSAO etc.) did anything noticeable, neither did changing the resolution.
My current theory is tesselation - I had it active on my 970, but can’t change the setting anymore. The Vega does support tesselation, maybe it’s still active and causes the drivers to select an inefficient render path… or something. The DX9 client doesn’t support tesselation, so that would fit the situation as well.
Tessellation was removed a while ago. There’s still a /setoption command for it (/setoption RenderSetting_Tessellation), but it does nothing.
It’s been a while, but I found the culprit, and just want to leave this information here in case someone else has similar problems.
The reason for the low framerate was that there were still some nvidia driver files left on my system inside the SysWOW64 folder, and SWL was loading and unloading nvapi.dll several times per second (apparently inside the render loop?).
Note that this happened even though the nvidia drivers were removed as “properly” as possible - meaning via the driver’s own uninstall option. There were no entries left in the apps & features menu, and no hidden devices left in the device manager either.
The problems went away after I manually removed the nvapi.dll.