When I write I typically have some kinda reference going on in my head and such a good familiarity with my DAW that I can whip together an idea for an entire song in a day........ Fine tuning everything is a pain in the ass, though. Making sure all the notes are on beat, EQing the kick just perfectly so it punches through the sub, removing all the mud, getting the levels right, getting the waveform to not look like a spikey mess of kicks and snares. All that stuff that seems like minutia is actually super important for a listener (even me when I listen) so you always get that shit situated correctly. Transitions, effect wetness, EQ, compression, pannnig, all very important. I won't release "trash songs" so the post-production stage of music is the most time consuming and boring. Every song is a new learning experience though so sometimes you gotta just release that shit and move on.
Also something I find frustrating is not being able to find good answers online, everyone seems to have the "right way" to EQ a punchy kick or compress a snare, but they're all different. You never know if they're mixing punk, or blues, or rap, or metal, or EDM which are all mixed in completely different ways. Since I'm into trippy electronic music I gotta search through "EDM" tutorials to help me get the kinda sound I'm looking for and if I'm lucky I'll find something sorta similar to what I want. You can't get a room shaking sub bass using jazz bass EQ techniques...