User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:82.0) Gecko/20100101 Firefox/82.0
Steps to reproduce:
I had opened the browsing history window and did a search. Then I selected all search results (1500+ entries) and deleted them.
Firefox became totally unresponsive for almost 4 minutes until the deletion process completed. Deleting 300 entries took about 45 seconds. My PC's CPU workload had only been at about 50 - 60%. And memory usage was at about 44 - 51%.
The deletion process should not bring the browser to a halt for such a long time. If I could keep on using the browser while it deletes the entries in the background I would not mind as much. In the past with older Firefox versions I also experienced such kind of slowdowns when deleting a large number of browsing history entries. However, it has never been as bad as it is now with Firefox 82.x. In that regard this version seems to be at least two times as slow as versions from one or two years ago for instance.