Quick answer: Python by itself is way too slow for heavy calcs. If the application can stand on its own, it's better to write it completely in C or some compiled language. If it truly needs to interface with Poser (controlling Poser parameters directly, or using the Poser display) then the Poser-connect parts will be in Python and the time-critical parts in C. There are a couple of well-established ways to make this connection; the best is called Swig. I've used this to handle large OBJ files. Python does "natively" include some rather odd and specific addons that work fast, but these are not well suited for 3d. Also, there are a couple of available packages for fast 3D action in a strictly Python app. Look up PyGTK for a start. I haven't used this, so can't say how well it works.

My python page
My ShareCG freebies