Also test some of the other speed sites to see if there is a pattern.
My "official" speeds are 100/10 mbps from Time Warner cable.
On fast.com I get 53 mbps
on testmy.net I get 23.5 mbps
on speedtest.net I get the rated speed.
These tests were done in Windows XP. If I boot Windows 10 (same PC) I get the also get the rated speed on testmy.net.
Not sure why it has an issue in XP - Perhaps XP itself is the problem.
I get these speeds with avast installed, disabling the shields doesn't make any difference.