Painting can be a thankless task. Apart from hours that can (and indeed should) be spent preparing, applying paint is not a job that can ever be hurried. Painting will take as long as it takes and attempts to hurry will only result in spillage, runs, etc.
The finished result will depend on the quality of the paint, the brush(es) used, the ability of the painter but importantly will directly reflect the overall time taken. So if you want great results you can’t just slap on a quick coat and hope it’ll look okay.
Think of the black-painted door to No 10 Downing Street and try to imagine what it takes to get a finish like that. For one thing the weather outside is rarely conducive to obtaining a good painted finish because:
Wind—will blow up dust on your newly painted surface.
Rain—will leave ‘pot’ marks
Hot weather—every flying insect in town will want to walk in, and then stick to, your newly painted surface.
Cold weather—the paint often goes quite thick …also the painter gets cold hands.