The windows *have* to be at the same resolution in order to get accurate mouse repeating. If you want the repeater regions to match up with different resolutions, what you can do is move whatever you're clicking on to the upper portion of the screen. Because the resolutions are different, the size and position of the things you're wanting to click on are going to be different too... but you can do it if you want.

The problem with wanting to play at 1920x1200 with 5 characters is that it's a huge resolution! You don't have to run all your windows at 1920x1200. You can set them all to a smaller resolution instead, via the Desired Rendering Size option. 1280x800 is the same aspect ratio, but smaller. This would drastically improve your performance, you'd just have to get used to the game looking slightly different (blown up).

When you get the resolution you want, there are things you can do in the game to increase your FPS. Drop video settings to lowest, drop view distance to lowest and point camera downward on all but your main, and so on.