FPS drop with two monitors?

And that's when I stream on Twitch and run the second monitor and have the chat etc. And run the game on the main screen, I get a drop from 144 FPS to something with 60 FPS. That's how it is when I run the game in full-screen mode, but if I now have the game in window mode, I get over 100 FPS, but it doesn't feel like 100 FPS, it feels more like what I know, at least very uncomfortable for my eyes.

The GPU is the GTX 1070, CPU the Intel 9700K, 16 GB DDR4 Ram.

Can someone help me there or know why?

This is because your other screen is probably only 60hz.

Yes, that's right, the main screen can do 144Hz and the second one can only do 60Hz… Is that really because of it?

Yes this is a windows problem

Well, then I know now that I will be able to run the chat on iPad or laptop in the future.