The biggest thing that helps me is something that I'm still working on, and it's kinda a combination of things. The first half is that I often go to details too soon. This means that I'm working on a finalized version of something before I've worked out bigger elements like proportions or even sometimes the pose. This means that I end up spending a lot of time on things that either don't matter or need to be scrapped. The second half of this problem is that I fuss about things that don't matter, like getting clean strokes on someone's head that I'll be covering with hair, or swapping between two nearly identical color choices before choosing a third. The reason why I consider it all the same problem is because it stems from the same thing, perfectionism. You're going to be your own worst critic, most of the tiny things that you feel are glaring errors won't be noticed by others. So the biggest way to speed up is to just not care about the things that only you would notice.
Another thing that really helps is stacking the deck in my own favor. Working at my best time of the day, eliminating distractions, having all the references and supplies on hand before starting, and so on. It makes a big production out of creating art, which means I'll sometimes procrastinate starting, but once I start I can crank things out much faster.
The third thing is to allow yourself to do the quick and dirty way. I feel like using transformation and deformation tools is cheating, so I end up redrawing things to fix them instead of just using the software to push it into place. But when I watch pros work, they use transformation tools constantly. Speeding up sometimes means getting off your high horse and just doing it the fastest and easiest way instead of the more pristine or elegant way.
I've found most of my hang ups by doing speed challenges. Trying to get things done under the pressure of an extreme time limit really brings out the bad habits I developed over time. The really funny thing is that although going really fast isn't so great for my enjoyment level, I've notice no difference on the quality of the end result. The biggest time waster of all in my art is second guessing every step, and it does me no good.
As for actual techniques to make things go faster, that will depend on what stage of the art you're working on and what tools you like to use. I know you were wanting to do coloring next, so I'll tell you a trick I've used for doing flats. Make a second copy of your linework, make it black and white instead of black and transparent, bring the contrast waaaaay up so there's no anti aliasing anymore, delete the white, block any gaps by making lines with the pencil tool, then you have a completely clean linework to color in where you can just use the flood fill, and you can fill or delete the lines at the end to get rid of the evidence. Flats are all about getting color down quickly, so getting things exact is just not required at this stage. You can do that when you're refining things later. If your linework is good you can also just select inside the area, expand the selection and fill below the lines, but that method is more trouble than just coloring by hand if your linework has a lot of gaps.
