The number of keys doesn't make much of a difference unless you are instantiating A LOT of that skeleton and consequently having to play that same animation a lot at once.
Curve keys vs linear keys probably makes more of a difference in performance. But even then, only when you have like a ton of them.
Every skeleton is different so you always have to test it with the skeleton you are using and with your target device.
Only keys that "fire" are event keys. All other key types are interpolated/extrapolated
meaning it doesn't matter if all the keys are on the same frame.