I select the object and hit space bar. Everything works as expected for the following 8-10 seconds of footage, then there is a dramatic slowdown. Like I'd go from processing several frames a second at the start of the process to a couple frames every few minutes.
Other than making sure that you are only using Rotobrush on the frames that are going into the final composition, making the Rotobrush layer the only layer in a comp, then rendering the layer after the matte has been created, or using procedural mattes or even some manual roto and masking to take care of part of the transparency, I do not know of very much you can do to speed up Rotobrush.
You might try an earlier version of AE. Some users are reporting problems with Rotobrush in the latest release.
Even with my most powerful system my average comp is one shot under seven seconds. Worked on one recently that was just over 1 second. On rare occasions when I need a transition between shots I will put two shots in a comp. My animations are never longer than a phrase or a sentence. I do the editing in Premiere Pro. It sounds like more work but I get more done in a day than I could possibly get done if I tried to use AE as an NLE.