That's an interesting insight. There should be another path though: visual imagination, which already runs at (roughly?) the same speed as visual perception. We can already detect the images someone is imagining to some extent, and with uploads directly putting images into their visual cortex should be comparatively straightforward, so we can skip all that rendering geometric forms into pixels and decoding pixels back into geometric forms stuff. If you want the upload to see a black dog you just stimulate "black" and "dog" rather than painting anything.