As above please clarify, plenty times theres a mixup between bandwidth conversions.
150mbps = 18.8 MB/s
So 1.2mbps = 146KB/s
Task manger maxed out at 10mbps = 1.25MB/s but only getting 150KB/s roughly, correct?
Your other pc @ 25mbps = 3.125MB/s, even that is lacking.
Have you done a speed test? Speedtest.net
Steam server bandwidth is random so maybe try another server.
Check your ethernet cables also. To reach gigabit, over 10MB/s all 8 wires in the ethernet cable are used. If one wire is slighly bad the speed is knocked to megabit speeds only using 4 wires which is below 10MB/s, to the range of your 2nd pc.
If condition of network cable is pretty bad might explain why your first pc cant reach a Megabyte.