If we say slow down, why do we say speed up and not fast up?

Question

it has puzzled me for ages

Answer 1

Because "to slow" is a verb, but "to fast" isn't (in that sense).

It's really that easy.

I'm guessing you've never studied another language properly, because if you have, you'd catch on to it easily.

Answer 2

It's all about the S's.

It's a conspiracy thing and is somehow related to the underground n**i agenda which is hoping to convert our entire language to nothing but words that start in "S", so sthere syou sgo!  :-P

Seriously though, it's just one of those things.  Like driving on a parkway and parking on a driveway.

Answer 3

Because the sentence flow doesn't flow when you say fast up, also You can speed up you can not fast up or down.  

Answer 4

cuz fast up sounds g*y, thts why