I've not played Kerbal for ages, and since my last adventure I've upgraded my PC to what is effectively a whole new system. It's a system that should run Kerbal with no problems, yet I'm having the weirdest issue that I've never experienced before in any game.
When the game is in focus, CPU and GPU usage is very low, resulting in a framerate of about 20 FPS, even on the main menu. If I watch my CPU usage on my second screen, I can see it sat at about 1%. But, weirdly, if I set Kerbal to windowed mode and switch focus to my second screen, I see framerate skyrocket to about 500 FPS while Kerbal is out of focus but still visible on my main monitor. CPU temperature goes up, as does GPU usage. I've tested every combination of settings (including various V-sync settings, frame locks, SLI on/off, etc) I can think of and the same behaviour happens every time.
I grabbed some video footage of this happening to demonstrate my point. Keep an eye on the info in the top left. You'll see it start on 20 FPS while the game is in focus, then shoot up to above 250 FPS when focus is changed. Bringing the focus back to Kerbal reduces it back to a solid 20 FPS once again.
Hopefully I've explained the problem. If anyone has experienced something similar or has any idea (no matter how noobish - I may have missed something obvious) what may be causing the system to use no resources to run the game while it's in focus, I'd really appreciate some help!
System specs are as follows:
Intel i7 7700k @ 4.8 GHz
2 x GTX 980 Ti
16 GB RAM
Samsung SSD 960 EVO 250 GB (where the game is installed)