in the test installation, 2 tag readers were running.
Anyway, I see your efforts and testing and I don't think we can resolve this issue within a reasonable time frame. I think I'll simply reduce my collection to be scanned by around 50K files that I have mainly for archival purposes; I can manage them elsewhere. With 20K files, Strawberry performs ok, as-is. I'll continue to test and will let you know when the larger collection gets scanned fine.
BTW: Thank you for the great work on Strawberry, I like the configurability and the easy way of retrieving good quality cover images. So, I keep using it. As I saw on github, somebody else has already posted a wish for multiple genres per file, which seems to imply a lot of rework with respect to the tag reader and the database. It's really the one feature that would make Strawberry perfect for me. Using picard and EasyTag for tagging, I like to keep my collection clean and well-tagged, and I'd love to see the player list all tags separately, in the tree. But that's another story, for another day...