And just like that, I think I found the problem.
I had g-sync enabled on windowed and fullscreen. Turned it to full screen only, set factorio to be windowed, and restarted the game, now it runs at a solid 60 fps.
Hope this helps for OP too!

I have the exact same problem!
Just got a new video card nVidia gtx 1080, and this has the save problem.
In the game, the fps is stuck between 22-24, but if I tab out, the game instantly goes to 60 fps.
I've also tried the nvidia control panel power settings, and the console command game speed. but ...