Xfinity throttling my internet then denies it.

I'm seeing something similar with the new XB7 gigabit modem. I'd suggest this--I upgraded to a Cat7 ethernet cable from a Cat6 and my speeds increased by 30%. Well worth it. I'll try the VPN though.

I have never heard of or experienced a vpn that conducted speeds in excess of 600 Mbps, what is the name of it and what protocol do you use, very curious 

One, concur, never seen a VPN(including work) that support 600mbps

two, that would mean thay they are throttling specifically the speedtest site because while on VPN they(probably) can't see where you are going but they could still throttle your traffic.

I've upgraded my cable to a Cat8, and I'm finally getting speedtest speeds on some sites of 800+Mbps, using the Ookla speedtest as a Chrome extension on commercial sites. However, while I can get 800 on usps.com and nytimes.com, some sites seem to specifically go no higher than 200Mbps. I'll bet there are many sites and speedtests that themselves throttle at 200Mbps.

So for speedtest sies, yes that happens

As for Some sittes not going over 200mbps, that its to be expected you can never (ok maybe not never)get gigabit speeds from a single connection to a singles web site.  Gigabit connections for consumers are really beneficial only with multiple users or you are downloading from multiples sites at once(like watching movie, downloading a game and working on VPN for work)

So I got this 2,5 gig usb Ethernet adapter, not throttleing me

What brand and model of adapter is it?

I got this one

https://www.vtop.shop/blogs/new-coming/drivergenius-2-5g-network-adapter

beware it had to run from a usb 3.0 port, if it's a usb 2.0 port, it only gets about 350 Mbps do to the limitation of the 2.0 port, i also have the Xb7 gateway, using the 2.5 gig Ethernet port on the back, cat 6 cable, 25 feet of it, Comcast just raised the speeds to 1200 Mbps, my equipment only had 1000 Mbps  network cards, so I figured why not take a chance, Amazon has a great return policy, a bonus is it works great in Linux also, so my old 1000 Mbps cards were maxed out at about 750+ Mbps, I assume it was because the limitations of the cards, I do have old wiring, I thought it could of been that, but it seems the speeds were always there,  I just needed the right network card/adapter that could easily handle the upgraded speeds, there are cheaper ones and more expensive ones

I just ordered an internal 2.5Gb card and upgraded my cable to a Cat 8. The switch from a Cat6 to 8 made a huge difference, and hopefully the new card will too. Thanks.

I will try the cat 8 as well, thanks for that tip, I was going to order a internal card but, both of my computers are from around 2011, I read tho that my particular computer the pci slots might not support that speed, that's why I tried the usb first, plus one is a laptop, let us know how the internal card works