What was Big Willy Shakes going for?
Richard is saying that people who wait around thinking and getting all bothered by little details like someone raising an army against you end up regretting it. He's more of the mindset that you should act instead of wait around for someone else to make a move.
For someone in his position, he pretty much has to. After all, he's killed everyone on his way to the crown, and everyone knows it, so he's got to go on defense it before someone else snatches it from his head. Put simply, he's telling Ratcliffe that he won't work at a slow pace and delay any longer.
If you want to get technical (and we always do), then you should know that a snail's pace is actually 40 feet per hour (source). Let's just stick with saying that's really slow.
We've also come across this phrase in Troilus and Cressida. We think Shakespeare came up with it and ran with it—even if it was at a snail's pace.