The trick Leonardo use to be this fast is that your screen only has a fixed amount of pixels and it is enough to update does pixels super fast for the application to feel fast. Then you have a lot more time to update the rest of the pixels.

This makes Leonardo kind of "resolution independent" and is always super fast whatever the canvas size and brush diameter. On top of this we also have a streaming mechanism of tiles between RAM and file so that you never run out of memory.