I described it here on the forum in some detail with quantitative performance measurements some time ago. If I remember correctly you also where a part of that discussion. Ah, found it: Performance Issues with Gramps - #16 by UlrichDemlehner
Basically the main point is that searches in the person and event views are just too slow and using SQLite as database backend in v6 doubles the time searches need compared with v5.1.6 on BSDDB. This is completely independent of using RegEx searches or “normal” ones. The other views are ok but those have entries in the range up to 10k or so and that has never been a problem with Gramps.