It can't be faster and there must be a small overhead somewhere to writing to both.
Nope. In any Raid configuration, the drives are writing
simultaneously. Ie. at the same time. It is not slower.
Any overhead that there might be when divvying out the
data is taken care of by the Raid controller.
I repeat: Raid is not slow!
There's a reason top-end gamers opt for these setups.