When doing speedtests within brave, I get a download speed of 40-65 Mbps. When doing it within the speedtest windows app I get a download speed of 980 Mbps. I get the 980~ speed when using MS Edge as well.
I get the poor performance with Firefox as well. I believe this is an issue being CREATED by microsoft to hamper 3rd party browsers.
I have not tried with a different OS as yet, but that is my next step.
While anything is possible, for me Brave just got better results on the simple fast.com test than MS Edge. (~70Mb/s vs. ~40).
I think trying with a different OS is useful, but maybe also look at the configuration of each of the browsers; for example, are you running a bunch of extensions and non-default ad blocker lists (e.g., brave://adblock) in Brave? And perhaps similar in Firefox – a bunch of extensions running?
Easy way to test would be to try with them all disabled, or using a Private window in each, assuming you don’t allow your extensions access to Private windows.