I talked to Apple Support about this issue last night, basically to ask a single question (I'd had at least four previous support sessions with Apple on this one issue): is it safe to simply remove the files from the two folders (on Intel systems; on Apple Silicon systems the second folder is inside the first): ~/library/metadata/CoreSpotlight/, and ~/library/metadata/SpotlightKnowledgeEvents?
The advisor said that Apple okays this approach, with the caveat that you should delete these two folders' contents, not the folders themselves.
On the two Intel Macs I was having this issue with, I deleted these two folders' contents, and saw immediate performance gains. For one thing, both systems had roughly half a terabyte of Spotlight metadata in these two folders, so I reclaimed all that storage space. For another, I saw a huge improvement in any kind of search that involves Spotlight: Finder searches, Spotlight window searches (invoked by default [CMD]-[SPACEBAR]), any searches in Mail, including smart folders; and quite a bit less processor usage by corespotlightd, which I believe is the process that writes out all this data in the first place.
That said, the problem isn't eliminated entirely. On one of these two systems, the metadata folders accumulated 8.4 GB of new data in literally a hour and a half (although it seems to have stopped growing at that point), and on the other system about 23 GB accumulated from when I removed the data last night until late this morning. But if this trick worked once, there's no reason to suppose it won't work again. So, unless the 15.3 update (or maybe some later update) addresses this issue, I'll just keep an eye on the ~/library/metadata folder, and if it grows to say 100 GB or more I'll simply delete this data again. As far as I can tell, that seems to serve no purpose other than to significantly degrade Spotlight performance. It certainly doesn't speed up searches; in fact, at about 500 GB of data, search was essentially halted in its tracks.
