In this case I think there's a big memory leak in R0J0hound's Canvas object in WebGL mode:
instanceProto.drawGL = function(glw)
glw.setTexture(this.runtime.glwrap.loadTexture(this.canvas, false, this.runtime.linearSampling));
It's loading a copy of the entire canvas in to memory every tick which is going to waste something like 70mb of memory every second, and eventually the garbage collector realises most of them aren't being used any more and throws them away. It's a bug in the plugin, the developer needs to fix it (@R0J0Hound: glw.deleteTexture()), but it is still inefficient, it may be best simply to leave WebGL off when using the Canvas plugin.
FYI creating and destroying objects a lot is inefficient - why not use one 'eraser' object and change its position?