Jean Fredette wrote:

Why would just switching the speedtest tab to "background" mode have any
"real" impact on the speedtest results?

Each browser manufacturer has its own reasons and viewpoint.

For example, here is Google's (Chrome) viewpoint:
https://developers.google.com/web/up...ackground_tabs

And this is a more technical view from Mozilla (Firefox):
https://hacks.mozilla.org/2017/06/an...om-scheduling/