I had this issue when i set up my second monitor aswell going from 300 fps to 40-100max, im gonna assume you use a nvidia graphics card and if you do this is how i fixed my issue.
1. Go to nvidia control panel.
2. Click adjust desktop size and postion.
3. You'll see it says "Perform scaling on Display" you want change that to GPU instead for both monitors.
I hope this helped you man.