So, Ant-man is noticeably slow when he is really big, when we see him fight.
Has a reason been given for this?
It's relative movement. Basically, the bigger you are, the further you move, but there's all sorts of things to account for, like drag (or friction), and gravity - without getting too technical.
You often see this kind of "slow" movement in other movies - Godzilla, Jurassic Park, etc., where all the bigger animals/monsters seem to move "slower" than normal. However, when you look at it from the other direction, as KutuluMike pointed out, things that are smaller than us move "faster". Can you imagine trying to move your limbs as quickly as an ant does?
My answer really is very much a Layman's answer, but if what I've already tried to explain doesn't make much sense, there are other conundrums (specifically within the Antman movie) that were simply explained away by "movie magic".