The browser does not influence your download/upload speeds, with the exception of Tor, because it sends your traffic through a series of VPNs.

I don’t know what you measured, but it’s not the difference between browsers.

Vivaldi is not privacy optimized.

Use Librewolf if you care.

And use Web Basemark 3.0 if you want actual tests.

Even though many, especially the canvas drawing stuff (where Firefox is horrible) are not really used anywhere useful.