So I recently replaced my i7-4790k CPU with an 17-10700k.

In Total War Warhammer 2 I got a framerate of 68 FPS at 1440p on my old CPU. On my new CPU I did a fresh benchmark and got 91 FPS. A great improvement!

However, some days later, I am gaming with my mate in this game, and I notice my FPS is shite. So I'm like WTH, close everything down, do a fresh benchmark, all settings the same. And I get 68 FPS again!! What the hell!?

It's been about a week since I've had my computer reformatted, so I've installed some programs, I'm guessing something is fucking the FPS, but I can't for the life of me figure it out. I've got up to date drivers, it's not running in window mode, I tried doing the benchmark with as many unnecessary programs closed as possible - I can't even get over 70FPS anymore!

Anyone got any ideas??

Interesting. I've had similar experiences in multiple games even without a hardware change and have yet to find a clear culprit. And it also started a couple weeks ago. Maybe some windows update?
I've even had one weird case where an old game that should run easily at 200fps dropped down to 20. The "fix" was running it in windowed fullscreen. So I also suspect my nVidia driver settings. Maybe unrelated, but in the past I noticed that in some games / configurations activating vsync drops the fps from well over 60 to 30 and a horrible input lag.

I always have vsync off. I've kind of moved away from suspecting my CPU now after I ran this test:

https://www.userbenchmark.com/UserRun/30401049

Which seems to be pointing at the GPU as the culprit.