When you set the blend mode on the canvas graphics context with setGlobalBlendMode it does the blend on the whole canvas size for each draw call not the size of the drawn graphic. So in my example I am drawing 100s of 8x8 pixel circles into a 1024x768 canvas and it is doing 1024x768 into 1024x768 100s of times once for each 8x8 pixel circle. This grinds the whole computer to a halt.

To reproduce I have attached a version of my app, I have made the default window size small so it doesn't kill the whole machine for me. If you make it big I find it very hard to force quit the application as the machine is so slow.