As a cyclist who writes, or a writer who cycles, or a dad who writes and cycles, I've learned a thing or two about the hardest part of any of the above: letting things simmer.
As a cyclist, I possess a primordial urge to slay the dragon on every ride. Sitting in or taking a day off the bike to recover doesn't come easily despite the fact that rest clearly leads to more productive, enjoyable training over the long term. Resting is quite simply one of the hardest parts of training.
Writing requires a lot of subjectivity to make it worth anything. In subjectivity comes, naturally, an absence of objectivity. Any writing that hasn't been evaluated objectively has a thee gazillion percent higher risk of being sloppy, insular, misleading, and terribly-written. As such, letting it simmer for a day or two allows the writer to revisit the work and, if it's still hot like a rocket, ship it. More often than not, the time away reveals obvious needs for improvement.
As a parent, I'd like my kids to be quick and tidy about losing a bad habit, learning their math facts, or not eating like they've just returned from a two-year stint on a desert island. Alas, that extra deep breath and conscious choice to do absolutely nothing about it pays dividends far beyond the more immediate style of laying down the law, which all too often is parent-speak for "wasting one's breath".
I've tricked myself into thinking that every result is better when its attacked with militant urgency. Call me older and/or wiser though because patience has seeped into my system, and I'm realizing the big dividends it pays. To get all those rich textures and flavors, baby, I gotta let it simmer.