Yep, you're right

I was having a little chat with Google of the slower web page loading with FF compared to Brave. Mind you, it's not that drastic, but is noticeable.
I mentioned my extensions, and Google pointed out that Dark Reader takes up a lot of resources. So, I ran Speedmeter 3.1 with these results.
With Brave and my extensions being, Proton Pass, F-Secure browser extension, Zoom, and Dark Reader with Brave Shield set to aggressively block trackers and ads.
With Dark Reader enabled, my result was 13.5. With Dark Reader disabled in Extensions, the result was 23.3
With Firefox and my extensions being the same as with Brave, but also using uBlock Origin (i.e. no "Brave Shield").
With Dark Reader enabled, the result was 9.93, with Dark Reader disabled in Extensions, was 19.4
I do realize with Brave I can go into Brave Flags and enable dark mode, but on this site it wipes out the upper right corner icons (they're there when I hover my mouse over that area) and some members Avatars look like a film negative, so I disabled it.
I'm sure most of you all knew that about Dark Reader, but just in case, I thought I'd mention it