Game runs slow on DX11 since 24.0 Mainline update

[Andvarja]

Prior to 24.0, I ran DX11 and my performance in all game areas was good. Solid, consistent FPS at moderate settings (no drops below 60 fps except momentarily on loading transitions such as the Fortuna elevator), no real issues on open-world. Buttery smooth.

Since 24.0, my reported framerate is still just as it used to be, but that's not what I actually see on DX11 mode.  It seems framey. There's no freezing, no spikiness at all, it just looks like I'm getting much lower framerate than I actually am, especially in open-world - it seems like things are animated by stop-motion, or in a flipbook, albeit rather quickly.

And, at least if I'm solo, things are actually slowed down slightly.  Timing is just wrong.  Animations are consistent and relatively smooth (weird framiness aside), but slower than they should be.  Everything happens at the correct time relative to other events, nothing's getting out-of-sync (at least if I'm solo), it's just...a bit slow. I feel like this slowdown may actually be the root cause of the framey, stop-motion feel of everything.

I didn't try multiplayer this way, but I can't imagine it would have gone well with the slowdowns, particularly if I was host.

Mobo:  ASUS M5A99FX Pro R2.0
Processor:  AMD FX-8350 8-core, 4 GHz
Memory:  8 GB
Video: Nvidia GeForce GTX 960, 4GB vRAM
OS: Windows 10 Home 64-bit

Drivers are up-to date.  Temps are great, system is clean (hardware and software) and fans in good order.  Not experiencing any instability at all, whether my system or the game.

The very first troubleshooting step I tried was switching to DX10, and that cleared everything right up.  But I feel like that's maybe just a workaround.  

I'm probably going to try a complete, clean uninstall and reinstall of my drivers later to rule out an issue with my own configuration, but that's about all I can think of to try, myself.