For a test, what speed do you get with a computer hardwired directly to the CM1000 cable modem (no router in the mix) ?
There are several factors that determine speed when above about 300Mbps. I chased the same issue for days and found the bigest issue is if the test site is using IPv4 or IPv6. I found my router can only acheive about 350Mbps using IPv6, but can get 950Mbps using IPv4. The older Xfinity speed test had an option to set which IP version to use, but defaulted to IPv6. I have since switched to DSLreports speed test which allows selection of several test settings in the advanced section. To get the full 1Gbps I would use IPv4 and search for a very fast router. Most consumer routers will not achieve 1G using IPv6. I just bought a commercial router but I haven't put it online yet as runs Linux and uses command line syntax for settings. This class of router usually does not include WiFi so you will need a WiFi router as well. As stated above, everything must be wired with good ethernet cables.
I see the new Xfinity speed test now allows selection of test server and IP version. Another thing I found was if your computer is attached to your network through switche(s), the economical swithes will slow your speed as well. I plugged my computer directly into my modem to get full spped when I upgraded to 1G. I bought a managed, full duplex 16 port switch to replace my cascaded $15 5 port switches. Now everything is home run directly to the 16 port switch. This also made a huge different during my 1G testing.
Just to add a little more info to this thread. I have 1Gb service. I use a DOCIS 3.1 modem and an Orbi mesh router system. My desktop PC is on the wire. Modem -> Router-> Switch -> Desktop. Up until about a month ago, I was seeing between 970Mbs and 1.0 Gb on the speed test. On WiFi (laptop) getting about 550 Mbs, probably due to back chanenl communication over the mesh system, etc. Very acceptable.
About a month or two, ago, the speed test dropped to between 490Mbs and 510Mbs (on the wire) and has remained there. No other changes to my network have been made. It seems quite likely to me that Comcast has re-distributed bandwidth and/or throttled speed, perhaps due to increased demand during the lockdow when more folks are not in the office (i.e. they are online, at home).
I've tested this pretty thoroughly, and I'd love to hear some other feedback or explanation. Right now, I can't think of one...