The game no longer lags in a full restaurant. Drawing a busy lunch hour took about 21 ms a frame, more than the 16.7 ms that 60 frames a second allows. It now takes about 5 ms.
The cause was the people. Each person's skin, shirt and hair are recoloured once and kept, but the browser stored those recoloured pictures in a slow way and copied them to the graphics card again on every frame. They're now stored where the graphics card can draw them directly. Nobody looks any different.
CHEF!