The reason it looks smooth is because I’m recording at a low framerate and speeding it back up. Bearing in mind that my PC is 11 years old and wasn’t even high spec in its day, I imagine it would run smoothly at a low subdivision level on a modern machine (don’t take my word for it though, I’m only assuming).
In 2.79, which I’m mostly using, I get about 6fps at 1 subdiv level, which is choppy but workable, and 35fps at level 0. Surprisingly in 2.8 it’s slightly faster at level 1 (~6.3fps) and slightly slower at level 0 (~30fps)
I’ll take that into consideration. 
Indeed, that was the main focus of this demo:
Granted there are only a few expressions on display here, but I don’t see why it shouldn’t apply to the rest of the face rig as well once it’s all hooked up.
I should mention at this point that I’m thinking of separating the face rig from the body rig to keep the base version somewhat affordable - in fact I’ve devised a plan to separate lot of things out, so they can be “added on” as needed. I might have to get some consensus on that.