It's to prevent reading cross-origin images through JavaScript. A canvas is considered tainted if it contains data from a cross origin image that was loaded without CORS approval. Tainted canvases cannot be read from by JavaScript.

I don't think this requires a 1000x slowdown but I'm not super familiar with this area of the spec. It should be possible to cache the tainted state of the canvas in a single bit, only updating it when adding new images to the canvas, and checking the bit when converting the canvas to an array.

For some reason every single pixel access must be authorized through CORS. I am not sure if this is in the standard or just shoddy implementation in all browsers. Try it yourself, read pixels in one non-CORSed image, and then in CORSed and wonder what went wrong on the drawing board. Some people bypass it via WebGL.