Mac mini late 2012

Most of my writing’s done on a Late 2012 Mac mini. I originally bought it because I was tired of having to replace an entire iMac just because I wanted to upgrade the CPU or graphics card. The plan was that I’d hook up a mini to a 27-inch Thunderbolt display and then I could just replace the mini every few years without throwing away a perfectly good screen each time. Four years on and arguably I’m overdue for an upgrade, but, before I do that, I might as well see if there are any easy ways to tune up the old mini first. Specifically, I want it to start faster; five-and-a-half minutes is just unacceptable! And since this Mac is on its way to retirement, I don’t want to buy hardware upgrades; I’m not going to just SSD my way out of this one. I want to know what app, widget or extension is making it so slow, and fix it.