Ah, as easy as that :) That's the issue! The times I tested just the laptop it was unplugged and when playing with monitor was always plugged in. I can see the sudden FPS changes when I remove the power, now I feel stupid, thank you :)
From what I’ve read, there is no hdmi to dp cable capable of 1440p 240hz yet so guess I’ll stick with 144hz for now. Does G Sync work with regular HDMI?