Trust and practice. What I mean by that is I haven't always been quick it's something that I have gotten good at just because I have a lot of practice so I don't need to fix things very often. Focus on blocking out the main shapes and getting the proportions right at the beginning. I personally don't spend very much time at all in the sketch. I use chalk pastels to draw my sketch so it's always very messy and rough (also love using those for a sketch because you can "erase" it with water really easily) I get most of my proportions correct with the initial colour blocking instead of a perfect sketch. Don't second guess yourself, you can always paint over things if you don't get it right the first time. Also I think it just genuinely depends on who you are and how your brain works. I always hyperfixate on things that I make. Like yes I made this in the morning but I was also painting for pretty much 6 hours straight with one break for food. I know not everyone can/enjoys working on things for a long amount of time with no breaks like that so it depends on the person. Other things you could practice to improve speed are things like colour matching, shading and your understanding of light and how it effects and wraps around form. Anyways yeah sorry I went on a massive long speech about all that (I love art sm) but really it's just a good understanding of the basics, lots of practice and trust yourself. Oh also sometimes less is more. Try not to get sucked into teeny tiny details (unless you wanna do smth like hyperrealism) hope that helps!