Roto brush - how do I speed things up - I read about 'Freeze' but unsure what this means....

I am trying to roto brush many small components in a 1 minute/30 second video - it takes forever - is there a way to limit the frames I'm working with or something? Does seperating and hiding a roto layer mean it's ignored? I read in other threads about 'Freezing' something - unsure what this means...

I guess I could render the content that I've successfully Roto'd, and then delete that part of it, but I'm not sure that'll help