So, tried to play CoH with Stumpy last night and got an incredibly laggy experience (didn't even get out of the menus).

It turns out that the Windows 10 Creators update does some things under the hood that breaks this early DX10 game, but if you do get problems you can do the following:

Create a shortcut on your desktop to the RelicCOH.exe file. In the Properties of that shortcut go to the Compatibility Tab and check "Disable fullscreen optimisations" and apply. Run the shortcut once and if that fixes things, you should be able to delete the shortcut and launch through Steam as usual.

(Credit to the Steam Discussion forum for that gem.)