I think I've seen this before, but sorry I don't remember why other than it might have been a high poly model, maybe from MakeHuman or something.
I'm thinking it might be corrupt data, so maybe worth just deleting the mesh and re-importing it, re-assigning it to the bones, and at least you'll still have the animation data.
Another thing is to try deleting some keyframes - ones you can easily reproduce, try and 'nudge' the data to avoid a potential memory leak or overflow.
I think that maybe your in the same boat as me though, should really be using something a bit better than CharFX - as much as I like it and appreciate how easy it is to use, I think I'm gonna try and figure out Fragmotion and look at the other options. For me the worst thing is assigning verts to bones on high poly models - it's good to have such exacting control when you need it, but CharFX is clearly not designed for high vertex counts, it gets so tedious and there are better ways.