Mac OS UI Slow Response
The best way (in my opinion) is to launch an application called Activity Monitor. You can do so by using spotlight (the magnifying class in the upper right hand corner), or by pressing Command+Space at the same time. Just type "Activity Monitor" and an option should pop up. This lets you see the CPU usage by process, as well at RAM being utilized, and a lot of other good metrics. This should help you get a good idea of what might be slowing down your machine.