Tried this, first with my install as it was and then with a clean installation and after first run of the game and both times it looked and felt like it ran at a very very low framerate, something like 20 or less. Thief 1 with TFix and everything is flawless, as was Thief 2 before applying T2Fix. I'm on Windows 10 with the latest updates using an i7 3770K and GTX1080 with the latest drivers. I'm trying to run this at 1440p. I re-installed the runtimes as well to make sure I have the required versions. I chose the "full" install. It's laggy like that even when in the main game menus so it's not really a matter of 3D performance or settings anyway.
Edit: Solved, it's a conflict with GSYNC. I have one of the GSYNC Compatible
Freesync monitors (the AG241QX) and keep it enabled by default for both fullscreen and windowed mode. Disabling it with whatever method when running the game (like choosing for GSYNC to only be enabled for true fullscreen applications, the Thief games seem to run in borderless windowed mode so it won't apply to that) gets the flawless performance back. Weird, considering it was fine before/for Thief, but minor.