Gimp does seem to slow down a lot if an operation would affect a lot of pixels. That's a 13.5 megapixel image, if you're editing a 5MP image, full-canvas operat

Gimp does seem to slow down a lot if an operation would affect a lot of pixels. That's a 13.5 megapixel image, if you're editing a 5MP image, full-canvas operations are much faster. But also, try zooming in on your new 13.5MP image and using a reasonably sized brush, say 20.0 (px?) to paint on it - it's pretty responsive. But if you switch to a gigantic, 200 (px?) brush, it gets a bit laggy (but still fraction-of-second laggy, not multi-second laggy, at that scale).