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.