Ask any experienced painter what separates a good job from a bad one and you'll hear the same answer: preparation. Rolling color onto a wall is quick. Making sure the wall is ready to hold that color evenly, and that every edge and seam disappears under it, is where the time goes.
Prep starts with repairs. Nail holes, cracks at corners and above door frames, dents from furniture, and old patches that were never sanded flush all need to be filled, sanded, and primed. Paint doesn't hide these things; a fresh, even sheen actually makes them more visible.
Next comes caulk. Gaps between trim and wall, along crown molding, and around window casings are where the eye goes when a room is freshly painted. Filling them properly before painting is the difference between a room that looks finished and one that looks almost finished.
Priming matters more than most people think. Bare drywall patches, water stains, and dark colors being covered with light ones all need primer, or they'll flash through the finish coats as dull or discolored patches. Finally, masking and protection: floors, furniture, fixtures, and hardware covered so the only thing that gets paint is the wall. When all of that is done right, the finish coats go on fast and look the way you hoped.