My plan is 1.2Gbps internet speed but I am only getting 930 Mbps

... if you connect with ethernet then you are capped at 940 mbps ...

The description in that article is a little squirrely. If using Cat6 cable with a 2.5 Gbps or higher network adapter, you should be seeing 1.4 Gbps or so with 1.2 Gbps service. If your network adapter is rated at only 1 Gbps or the cable is substandard, you're limited to 940 Mbps due to the nature of Ethernet.

... But it's the same on wifi ...

It isn't clear what you are comparing here. The Ethernet speed should have nothing to do with Wifi speed.

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@BruceW​ I don't know, that article is direct from Xfinity. It states that I am limited to 940 if I use an Ethernet cord, so I can only assume that I can only get up to  940 because I am using the cord. So if I use wifi it should be higher, at least what I am paying for. So I disconnected the cord and used wifi, and the speeds were the same. I'm just trying to figure out how to get the maximum speed.

... that article is direct from Xfinity. It states that I am limited to 940 if I use an Ethernet cord ...

The article is oddly worded, out of date in spots, and just plain wrong in others. Note that the section you quoted is "For speeds up to 1.2 Gigabits per second", yet it specifies a "10/100/1000 Gigabit Ethernet interface or adapter for wired connection", even though faster adapters are available and required to deliver full 1.2 Gbps speed. The article should specify this. And the section on "For speeds up to 2 Gigabits per second" specifies fiber optic adapters, even though most areas where 2 Gbps is available are not using fiber optic cable.

As I say, the article is badly out of date. You're so right, the article is indeed "direct from Xfinity". A competent organization would have correct information in an up-to-date article.

How are you measuring speeds? What the model number or speed rating of your Ethernet adapter? Of your Wifi adapter? It isn't unusual for these to be only 1 Gbps or slower devices, even though you need something faster to get the full speed of your 1.2 Gbps service.

Are you using an external router? An Ethernet switch?

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I'm using Xfinity's XB8 modem. Killer E2600 Gigabit Ethernet Controller, it is 1gbps. Does that mean I need a faster ethernet adapter?

... Does that mean I need a faster ethernet adapter?

Yes. Recommend an adapter that can operate at 2.5 Gbps or higher. The one you have limits your Ethernet to 940 Mbps or so.

Thanks. I have a Dell, and it doesn't seem to have a faster adapter than the one I have. 

What is your Dell's model number?

It's an XPS 8940. I'm guessing it's not good enough for 2.5

Looking over Dell's specs on their support pages, it seems like a decent box, and only a few years old. I'm just guessing, but I suspect it would handle a 2.5 Gb Ethernet card.

Also according to Dell, the Wifi module should be either a Wi-Fi 5 Qualcomm QCA9377 (DW1810), for which they say the transfer rate is "Up to 433 Mbps", or a Wi-Fi 6 Killer AX1650i, transfer rate "Up to 2400 Mbps".

Yeah, mine has the 5 Qualcomm QCA9377. Dell doesn't offer a better ethernet card for my pc, at least that's what it says. I found one, Asus XG-C100C, 

that apparently works on my pc, but I have to do disable, or something, with Killer. Well this just got too complicated for me.