19 days ago we released nopCommerce 3.80. It has a critical performance issue. "PagedList" class was refactored wrong way (IQueryable was replaced with IEnumarable). Hence all records were always loaded into memoery. That's why today we have released a new version with the fix. So if you downloaded nopCommerce 3.80 before August 22nd, then please download the latest version again here (its version is still 3.80). Or you can manually apply the fix:
Please see this commit
Thanks a lot to Frederic for reporting