Downloading files causes the browser to hang my system for 10 seconds

Re the following info, the idea is, attach instrumentation, learn over time, what you are seeing - reported by each instrument, and notice patterns . . . and then your eye catches something: a difference: a different pattern from the norm.

That is R&D; lots of study and frustration before stumbling upon a bit that leads to discovery:

PS1. Make sure that you back up bookmarks, passwords as a matter of routine. (Use Brave Browser’s Bookmarks Manager - Export . . . button, to make “bookmarks.html” files and save those files some place NOT within any of the Brave Browser installation.)

PS2. Create System Restore Points (you can police those by using CCleaner).