If you install that garbage "gamerfirst" stuff asus bundles with the motherboard try to uninstall it. It provides no real benefit since it can't affect traffic outside your machine. It causes all kinds of strange errors for some people.
There are 2 fairly easy things you can try. You can get a USB linux boot image and boot that. It will not affect your machine since it runs from the USB stick. The generally have something like chrome preinstalled so you can just run the same speedtest you do under windows.
The other is you can load a old line mode program called IPERF on 2 of your machines. It tests very basic network transfers between machines. You should get 900+mbps in both directions since it is not affected by...