I think the PCs are the limiting factor. but none of this explains the vast difference in the various speedtests between Fast.com, Thinkbroadband and Ookla. All were consistent when I had FTTC (51Mbps)
Nobody has asked what security product you have installed, if any. Many of these cause chaos with higher speeds as they are forcing everything you do through extra software. In many cases this software is only 1 day old and badly tested. Many major corporates have abandoned third party security software.
Your Phenom II is probably 9 years old, and if running security software may give slow speeds. For example my Core2Duo laptop only just manages my 200 Mbps virgin media connection, but then its 12 years old!!
Speeds faster than 500 Mbps can show problems in network card drivers. Which Windows version are you running, and what is the network card? You should disable any WiFi when doing testing using Ethernet, although normally Windows will prioritise wired.
Your i5-6500 should easily handle the bandwidth, but depends on the network card, is it built in to the motherboard, and which make and driver.
21 years of broadband connectivity since 1999 trial - Live BQM