A buddy of mine is more tolerant of computer problems than I am, but he's been dropping hints that he'd like me to take a look at his Mac Mini. He's an AS/400 guy, not a Mac geek.
So I dropped by and did the usual hygiene on his Snow Leopard Mini (SL is a great OS, so no rush to change that). Turned off Spotlight (make boot disk "Private"), restarted holding down D key to run hardware test, restarted holding shift key to run Apple's Safe Mode (startup volume check), ran software update, checked Flash versions, ran Onyx.
After that it seemed fast and smooth. So I restarted Spotlight and saw "Estimating Indexing Time" in the Spotlight drop down. Not just for seconds -- for minutes. Spotlight was stuck.
One solution, is to move all 3rd Party Spotlight Importers out of /Library/Spotlight username/Library/Spotlight. He had 3: iWork, Office, and Parallels. I moved them all out and Spotlight quickly estimated remaining time and completed indexing.
I think his months/years of performance issues came from Spotlight constantly reindexing/getting stuck. Naughty Apple -- there should be a better way to deal with flawed 3rd Party products. (Maybe there is in Mavericks.) If I had to bet I'd wonder if the versions of Office docs created by Open Office somehow cause problems for Microsoft Office's mdimporter.
That's quite a bit of suffering for a bad 3rd party Spotlight indexing tool he didn't know existed. It's also illustrative of how hard it is to maintain a modern computer; what was a minor cleanup for me had stalled him for months. One reason iOS is so popular -- and so closed.