Hey! Great job on the Gentar, and the One Punch Man and Pokèmon guy in the other post!
I got myself a Wacom Intuos Pro M a couple months ago and went straight for Krita too.
As for how to paint faster - yeah, draw things on different layers - the lineart, details, paint, shadows, highlights. Start big, erase what's not needed. Also on the layers there's an option to lock alpha channel (so that you only fill what's visible, not the transparent part) which should be a great help when doing shadows and highlights, etc.
If you haven't before, I suggest watching Aaron Rutten's tutorials - he has a lot of them starting from graphic tablet reviews and suggestions to different digital art software tutorials to basic drawing tips to whole "how to draw / paint this or that" videos.
Oh, I almost forgot...
Domestika offers free online creative courses on all kinds of art during the quarantine!