OK, seems most people here haven't experienced this problem before
What you're most likely experiencing is a Fillrate bottleneck. Basically, a GPU can only draw at maximum a certain number of pixels every second. On desktop PC's this number is usually in the hundreds of millions  but at 1080p with a few layers of full-screen graphics, you can run into the problem very quickly.
1920 * 1080 ~ 2,000,000 pixels
2,000,000 * 60fps = 120,000,000
Now multiply that by how many fullscreen graphics you have, layers with shaders or 'force own texture' on, etc.
The solution to this is either to reduce the amount of fullscreen graphics, or just choose a lower window size and set fullscreen scaling to low quality.