Hello Paul Jackson, indeed and this is not due to the add-on, but to Firefox API.
See this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1396364

This is hitting peope with lots of bookmarks. Firefox devs seem to think that 95% of users have less than 2000 bookmarks, and they consider people like you (or me) as "pathological cases" :-)

Patiently waiting for things to improve on Firefox side, and trying some workarounds meanwhile .. https://github.com/aaFn/Bookmark-search-plus-2/issues/2

Sorry that I do not have a better answer, aaFn.