I only have this issue with steam on CachyOS, Windows gives me up to 700Mbps, while on cachy I’m stuck at 120Mbps, the best results I got was 300Mbps by playing with the configs in the steam console, downloading from say the browser or other applications works just fine reaching the 700 range I see on windows.
I made 300Mbps stable by disabling ipv6 (my isp doesn’t support ipv6, wondering if disabling ipv6 on kernel command line will make it even better).