Yes! I have to agree that it takes very long to save images...
which are "BIG" ... but I am quite happy for that I am able to
paint and sketch quite large scale..

we are using mobile device and limited memory size and
if I can work quite smoothly with 5MP images.. it is enough for me
now and I can enjoy sketching, drawing and painting.

I now tested very big canvases with many layers.. and yes
it takes too long to save and load to work with conveniently.

Ok.. I now understand that it can take forever to save "very" large
images... .. I rarely use much more than about 3000x3000..
very rarely... still I have no 20 minutes saving and loading times..

My workflow could be so different maybe I have used so few strokes. I dunno know?

But keep going making some art... even in small scale..
so that you do not have to worry about loading and saving times..

I think it still enough and more than current mobile sketching devices are offer to us..

Tone aka Creativetone aka Luovatone