Morphs are just numbers, one set per vertex. In fact they are the deltas on your vertices, ie the numbers that need to be added to move vertices from their original positions to the positions in the morph (as the dial moves from 0 to 1)

The problem is Firefly only understands polys, it doesn't know anything about morphs or conforms or magnets or whatever, it just wants a plain mesh it can go to work on. So before poser can actually start a render, it has to take all those morphs etc and apply them to your mesh so that it's 'baked' into a form that can be sent to firefly. This of course takes CPU and RAM and if there's too many things going on, can really kill your scene. Even worse the process of baking is not multithreaded, so it only ever uses one core of your CPU which makes it even harder and more unstable.

This also jacks up your file size as with each morph, the list of numbers to be stored in the scene grows and grows, which of course takes longer for poser to read and arrange in your scene and to compress and write out when you save.