Things are a few things I've learned trying to do warmup sketches using a pencil and paper:
1. Sketches on paper look a lot better on paper than they do on a computer, tablet, etc.
2. Sketching with a pencil involves a LOT of erasing with an eraser.
3. There is no easy way to clean up lots and lots of eraser bits.
4. Sketching on an iPad is MUCH cleaner.
So I am going to stick to my same requirements (two minutes to draw, three minutes to color) for my daily warmup sketches, but I am going to move the process over to an iPad using Procreate, both of which are pretty awesome.
I'm not sure either the hardware or the software will improve my art at all, but at least I won't have to keep wiping little bits of rubber off my notebook, pants, couch, coffee table, etc.