IIRC, running this against just a couple of books it is run as part of the main Calibre process, but select several books (user configurable threshold) and it spawns a background worker process.
Oddly, I had several issues with memory leaks while running as part of the main process, but the spawned background jobs have always been well behaved on my machines. But I'm almost all epub & mobi files.
I wonder what would happen if you raised the threshold in the plugin configuration and tried that same group of 10 as a foreground process instead of as a background process ....
And are your books pdfs? Or ....?