It's not the aspect ratios that would slow anything down, the difference in aspect ratios is what makes it look bad. The slow-down would be if it were swapping using different rendering resolutions instead of preserving them (which as you're observing keeps it fast but is making it look bad by fitting it to your window sizes with a different aspect ratio).

ISBoxer 38 (currently in beta) adds new Window Layout styles to the wizard that will maintain the aspect ratio by making the windows smaller than 1/4 of your new screen, as well as various other options that are helpful for multi-monitor setups.