I think it's meant to be a way to "balance" the augmented speed attack

Imagine a Mage: Large wait beetween attacks but medium-length animation. If you speed the animation you get a tiny better DPS; if you speed the wait time you end up with a bigger DPS than the shorter animation and a lot better than the original

Imagine a Healer: Short to medium wait between attacks but long casting time. If you speed the casting animation the DPS goes up a lot: if you speed the wait time it's not gonna improve that much since it was smaller to begin with

I know it's not implemented this way, but what if they thought it like a mechanic to balance something that could get easily off the roof?

Then again it's a fantasy-tower defencec-ero-game, let's not burn our brains over it :P