Don't do stuff like pushing rectangles to an array to have their position wrapped, instead of simply doing it right there, unless you want to test the GC.
Yeah, by default it's accelerated anyway, at least if depending on browser and hardware. So, at least for something as basic as drawing a lot of things with the same color, engines and additional code can only make it slower.
Generally speaking, avoid GC, and avoid setting state that's already set... if you do these two things and use engines that do these two things, it's usually going to be more than fast enough :)
Its nice if you can avoid creating new objects, but in recent optimization I've seen no difference between reusing objects vs immutable objects. Removing unnecessary code is always nice though.
Don't do stuff like pushing rectangles to an array to have their position wrapped, instead of simply doing it right there, unless you want to test the GC.