Rotobrush - difference between hitting spacebar and hitting freeze button in terms of computation?

Hi, I see that if I am in rotobrush layer and hit space bar it propogates all frames.  For a given project I see that process took about 80 seconds.  If I don't hit spacebar and just hit freeze, then that also seems to accomplish the propogating and takes over 3 minutes.  

Trying to understand in which process is the required computation taking place.  I though freezing was an optional step that just locks in the result, but is it more than that in terms of final computed result?  Is the extra time for freezing just the cache'ing going on?

thanks