CM 500 is slower than xFinity xFi Gateway

[CM500-onXfinity]

Using my purchased NetGear CM 500 Cable Modem my XFinity speed is much slower than with the xFi Advanced Gateway.

NOTE: The CM 500 was fine for me for more than 2 years - but now apparently it's much slower than the newer xFi Gateway.

With NetGear CM 500 I get max speeds of around 28 / 9 [Down / Up] and 7-8 ms latency.

After switching to the rented Xfinity device [xFi Advanced Gateway integrated CM + Router Model Number: CGM4140COM] my download speed jumps to 400 / 15 [Down / Up] and 6 ms latency. [same setup, same Cable drop, same day but different hour]

In all cases I'm connecting to the router/gateway via WiFi - [I have no ethernet port on my laptop so Hardware is not an option - my testing methodology admittedly isn't perfect.]

Is this only because the NetGear CM 500 Cable Modem is DOCSIS 3.0 whereas the rented Xfinity device is DOCSIS 3.1 ?

Do I somehow need to upgrade the firmware on the CM 500 [which apparently can only be done by the Service Provider?]

How do I even check my CM 500 firmware to see if it's an issue?

If I upgrade to a DOCSIS 3.1 Cable Modem would I likely achieve the speed of the rented XFinity device or is there something else I'm missing here?

[plemans]

The CM500 is capable of more speed than that on xfinity. 

What router are you connecting to it and what band are you testing on (2.4ghz is slow)

That's kind of key. 

I'd find some type of usb/ethernet adapter to test wired speeds. Many things impact wireless.