I've been running better the past few days. I'm still too worried to call it a definite fix and I can't tell which of my recent experiments worked, but could you try those for me and tell me if they help any?
That is, if you also have win10 and nividia working (or in that case not working) together. They really seem to hate dx11. I'm not qualified for tech support, but I did contact IT for my issue, and it seems to be a display driver issue.
So what I tried last was:

1) Uninstalling nvidia gefore experience: I was reading in a Path-of-Fire dx11 topic that even when you close the programm, it still messes with win10

2) Disable Vsync (this is what I expect helps dx11 the most) and select Maximum Performance in Nvidia Control Panel
Right-click on the desktop area then click NVIDIA Control Panel.
Now click on Manage 3D settings.
Set Power Settings to Maximum Performance and turn off Vertical Sync. (ff doesn't let you disable vsync, that's why I did it via my nvidia panel.)
Click Apply to save changes.
Right click battery icon on the system tray and select Power Options.
In the Power Options window select High Performance under Choose or customize a power plan.
Reboot your PC to save changes.

3) Change Power Management Setting (that's what I found when I looked for display driver fixes instead of dx fixes)
Go to Control Panel.
View by “Small icons” in Control Panel and click Power Options.
Change plans to High Performance. After opening Power Options window, if you do not see High Performance option, expand “Show additional plans”, then you will see it there.
After that, click Change plan settings.
Click Change advanced power settings.
Expand PCI Express then Link State Power Management. Make sure the setting is Off.
Right-click on PC desktop and click NVIDIA Control Panel.
Click Manage 3D settings in right pane. In left pane at “Global Settings” tab, select “Power management mode” to Prefer maximum performance.
Then click Apply button to make the change take effect.
If the problem occurs while you are playing games, please also:
Right click the the game icon and select Properties. Select the “Compatibility” tab and check the box for “Disable Desktop Composition” and click OK button. (<- I couldnt find this for the steam version, but I copied it just in case.)