I know it's possible to see the fps of a program via some overlay like mangohud,but that doesn't work on all programs (for example firefox,and even if it would work for that it would be dumb cause you couldn't navigate the tabs properly) and it is also annoying to use an overlay like that,it would be just cooler to open the system monitor on a second screen or virtual monitor or whatever and just look at how many fps each program has.
Interesting idea.
There are some technical issues. Firefox will only repaint when it actually has new content, so if you get an FPS of 1, is the app nearly frozen or is it just the text cursor blinking in a text field. I can't see how it would provide any useful information. We also have a cardinality problem. One app has 0->infinite windows. So showing one FPS is useless.