when i start rendering, mplay takes about two minutes to pop up, and rendering so slow, what's going on?

There are a lot of stuff done before the actual rendering is started.

For example preparing geometry and rendering shadow maps. This could be the reason for the delay of mplay opening.

Might also want the what machine you're working with. /wink

eitht.

try disabling all network devices and see if that helps

If your scene has heavy image textures you could try converting them to .rat otherwise it needs to be done implicitly by Houdini.