I had this exact issue then found out either my cable or port on my router was bad, and was only initiating a 100mbps link. Steam can hit 40MBps (over 300mbps)

I had this exact issue then found out either my cable or port on my router was bad, and was only initiating a 100mbps link. Steam can hit 40MBps (over 300mbps) regularly for me. I find what you say to be true though, most services can't saturate my link - but I'd double check because exactly 100mbps is suspicious. Also don't use speedtest.net, try fast.com (run by netflix) - ISPS are known to "cheat" on speedtest.net (they can host a speedtest server inside their network) but fast.com downloads actual movie fragments from netflix's cdn.