Been fiddling with this for the past hour or so. Pretty impressive stuff. A couple things i noticed:
In the drawing I just did (which was a heavily graphical image of a persons face - don't know if that makes any difference?) It was just drawing every line 3 or 4 times. Maybe this is something to do with resolution or because i was drawing on a standard wooden sign instead of a large picture frame but completion time would have been a third if it didn't have to do this.
I also found that sometimes hitting f12 is not very responsive, you have to bash it a few times to get it to react, not a huge issue though.
Only other thing i noticed is that it quite often draws over itself, I don't know if that's just a failing of the games accuracy with drawing but obviously drawing over an area that's already been done increases draw time.
Other than these things I think it's pretty awesome!
Keep it up :)
Max