There is no option to limit the search area for the image searching.
Currently the only things that would help is a smaller screen or a faster Mac. Not having Keyboard Maestro editor displaying any Find Image actions will help somewhat since it will be periodically asking to search the screen to update the action displays as well, and that will add more load to your processor.
The ability to limit the search area is on the todo list, but I have no ETA on when that might happen I’m afraid.
The only other option would be to use some other tool to find the images (eg screencapture the screen and then ask another tool to process the image). I have no idea what tool might do that though.