Hmm. The key here may be the statement that things seem to render fine as a still. Take a look at your material settings, and see if they have been set to animate. Just changing the transparency setting once on any frame past 000 will create a keyframe (There is also a tiny key on the shader settings that toggles keyframing on and off; make sure it's off), and if you did any copy-paste of materials, it can be easy to set a value wrong, and then plaster it all over your figure...with interesting results. The reason it's slow is due to the large transparency that is being rendered; particularly if you are using raytracing.