Thank you for the feedback. As far as I know, the extension works OK with 20k bookmarks and up to 1m history visits; about a 50MB places.sqlite file.

You may experience a slowdown when making changes using the Library with a large (more than 20) number of tabs open concurrently. I will look at mediating this problem in future versions.

You can always open an issue here: https://gitlab.com/clu-os/firefox-add-ons/Related_Bookmarks/-/issues