only getting half the speed i am paying for

For the past few months i have been experiencing slow internet speeds that are only around half the advertised speed that I am paying for. I have the Gigabit internet only plan but only ever get ~500gbps or even less. I have spent hours chatting with agents trying to get my problem resolved but all they end up doing is restarting my router way too many times. i have run countless speed tests on the xfinity speed test, speedtest.net and the google internet speedtest and they all give me the results being 500gbps or less but when i do on the xfinity speed test the results come back that its at 900+. i like game alot and even sometimes broadcast my gameplay so I need the Gigabit internet to have the best and smoothest experience possible with the least possible lag but thats just not the case and i feel like im being ripped off paying $120 for Gigabit and only getting half or less.  anyone know how i can fix this?