the fill tool is your friend. do your flat colours w a brush thatll fill easily without too much hairiness
the clipping group and mask tools are also really your friends. i tend to use a masking layer to fill in the whole of my characters, and then i can be lazier w the acc colours. saves time on careful strokes and erasing.
also, flat colours + effect layers is way faster and often more effective than doing full shading. personally, i like doing a layer on csp soft light (or lumi & shade w low transparency on sai) that has a gradient from light and dark, then get a pen and draw in the hard shadows with the dark tone. an overlay over that is a nice touch up too