I'm not sure that it's slow moving. It's just that it generally grows on the edges, and unless you're out near the edges, you've no idea how quickly the edges are growing.
Fair enough, my view of mathematics is very much the lay persons field, seeing the mainstream mathematics as mostly unchanged for the last decade or two, but for someone doing mathematics professionally in a research capacity I would expect them to see nothing but green fields and novel approaches. And as the field grows (assuming a two dimensional, spherical math field here for the moment) so do the edges.
