I used to run Doom 2 on my 386sx. Doom 2 required a math co-processor, which the sx didn't have, so I had to use a TSR which emulated one, slowing my CPU down even more. It probably ran at 1-2 FPS, but it was "playable" enough for me to spend countless hours playing it. Ahhh, memories...
So yeah, 20 FPS is definitely what I would call playable. ;)