It's hard to tell what You could do faster, without knowing what and how you do and how much time it takes.
As for the tools - I'm using Windows Paint - and the old one at that (got newer version installed on new computer, but then I installed the old one alongside, as I preferred the way it worked - only thing I'm doing on the new one is the color customization) - and working on it is sufficiently fast for me:
![[ img ]](https://www.dropbox.com/s/2xhmisjyesdbe0u/paint.png?raw=1)
In terms of drawing itself - it's good to have prepared palettes of standard colors in various shades, as well as contour-only samples of roundels (airline logos etc.) - you just have to prepare them first time you need them, and then - provided they are of approximately same size (within accuracy permitted of the scale) you can copy-paste them (or just modify, instead of doing from scratch). And generally I always make the drawing in false-colors and just repaint them by left-clicking the color to be replaced, right-clicking color that ought to be there and "brushing" the area in question.
Using lots of copy-paste also helps - when making a series of the aircraft of particular type, that happen to share similar camo, you may want to have one "pre-painted" and then copy it and apply different markings on each, or just have various sections of paint scheme copy-pasted and just applied as needed (for example if they appear in various combinations).
(random sample)
![[ img ]](https://www.dropbox.com/s/xlgv0wv3uieexpi/sample.png?raw=1)