hi, i am having the exact same problem, it started with a win10 update, the speedtest and fast.com shows my speed right, i have 60 down 6 up.
all of the launchers i have, like steam, rockstar, epic, shows download speeds at 8mb/s thats the 60mbps, it just happens in the web browser, i´ve contacted microsoft about this problem, they just say that it´s my ISP but how do you explain that in those launchers the speed is the actual internet speed that i pay for.
anything that i´ve found in some forums is that flush dns and change it for google´s will fix it, but no.
i have win 10 lastest updates, PC´s with ryzen and intel 1gbps ethernet cards, and before you tell me to check if the link is 1gbps thats check.
cable 5e 1.5m
16 ram
it happens in edge, chrome and firefox, i´ve tried downloading from ms servers, google drive, mediafire, mega, and all that servers shows max speed of 1.3mb/s or 10-11 mbps, i can download 2 or more files at the same time and the speed will add up, so if i download from 3 different servers, 3 different files, the speed of each is 1.3 but the sum of the 3 will be 3.9 or 4, so the bandwidth is there, just that the file is at thas specific speed, and i dont think reinstalling win will fix it because i have the same problem in two differnet machines with win 10.
before the win update i could download any file of any server at full speed, this just started with the win update, i have the google dns, reinstalling drivers, and nothing helps, the only thing that´s good is that at least the steam launcher doesn´t got affected., thanks i hope someone can help us and not just give us solutions that we´ve already tried, thanks