Roto Brush rendering taking far to long

5 years late but I just faced the same problem and I fixed it by freezing (with snowflake icon beside it)  the roto after brushing the area that I wanted. It will reduce the render time and you can export it normally.

Are you using Media Encoder to render?
I had a 7-second clip that took 6 hours to render with Media Encoder. However, the same clip only took about ten minutes to render in After Effects."

Mine is taking 5 hours for a 10 second clip. I had my cache files checked and cleaned and changed my bitrate to a lower setting about 10MBPS. 

There is also nothing preventing you from splitting up the layer you are trying to roto. If Rotobrush takes longer than a couple of minutes to propagate and to speed up my work in general, I will use Shift + Ctrl/Cmnd + D to split the longer layer into sections that are only a few seconds long, run rotobrush on each of the layers, freeze rotobrush, then pre-compose the entire stack of layers and use that as a track matte. 

I also strongly recommend that you use the Composition/Prerender menu to render and replace your Rotobrush layer if the shot is at all complicated. Then you can remove the rotobrush comp's from the project, greatly reducing the file size and at the same time, improving rendering efficiency and reliability.

For most people it works in 18.0 and 18.1, but a lot of them report issues in any 18.2.x, so you may indeed need to go back to the original base version from last year.

Mylenium

It's apparently extremely buggy in the latest versions, so you may want to consider going back to an older version.

Mylenium