Is it faster to have an object listen for a custom event rather than, let's say, having the same object check for some variable to change every time it enters a frame?

For example, if certain enemies only target you if you are jumping, would it be better to dispatch a custom event whenever you jump, or have the enemies check your location every frame?

If using a custom event is the right way of doing it, could someone please give me an incredibly simple example of how to set this up in code? I'm trying to figure out how to use custom events, but all the examples I find are for crappy applications, so I'm having a little trouble wrapping my head around this stuff for the first time.