why is kha'zix q on such a low cooldown?

can someone explain the logistics around this. 4 seconds before cdr, like 2.3 after, 1.2 seconds if he hits an isolated target. that's some serious dps on an "assassin" for pretty much free, and it's quite long range as well. he's not only an assassin that deletes squishies, he chugs down tanks with ease and his ridiculously high ratios.

I just don't get it isn't his kit oveloaded enough?