It was slow. I just did an experiment with Ghost Shooter and (GS) Rain Demo.
GS: PC 60fps, Mac 60fps, RPi 30fps
RD: PC 40fps, Mac 30fps, RPi 5fps
I tried your Monsters in the Night and it was OK, but noticeably not as quick as you'd expect.
Personally, I'd say if you have no other use for the RPi, I wouldn't bother. Especially given the newest OS update makes WebGL completely unusable.