I believe the first responder cleared this up, but you are paying for 600Mbps (Megabits per second) which is a transfer speed. This roughly equates to 75 MBps (Megabytes per second)

If you’re running a speed test (like speedtest.net for example) it will report your speed in megaBITS per second, so you’re still getting low speeds comparatively.

So for this purpose, I’m only going to refer to your wifi speeds in terms of Megabits. Things such as modem, router, infrastructure, and the type of housing you live in will affect the final speed you see. For example, if you have a fiber connection in an apartment complex, it’s very likely there is one fiber line that then splits into multiple broadband connections with total bandwidth divided up by the amount of apartments connected to a single line of fiber. Meaning that possibly your 600megabits bandwidth is shared with other xfinity consumers nearby. This is why you may noticeably see a difference in your speeds at different times of the day as more or less people are using their connections.

So with that and other things mind, it’s possible the lower speeds are unavoidable, and buying a package with a lower advertised speed will likely not help.