slow internet speeds

I've been dealing with slow internet speeds for over a month now.  I have talked to various people over the phone support to little avail.  I have premier service with up to 100 Mbps, I obviously don't expect to get close to that, however I'm currently only getting about 25Mbps down on average.  This seemed to start after I cancelled my tv package, I didn't really ever watch tv so I decided to simply cancel it.  I'm wondering if perhaps when they shut that off that perhaps they also accidentally downgraded me?  Someone from the phone support sent out a tech to check the signals (since they were in fact off) and he managed to make sure all the signals to the modem were within range.  So despite the signal getting better, my internet speeds unfortunately did not.  I work in the IT industry and know my computer does not have a virus, I actually just completely wiped and did a clean install of it 3 months ago when windows 10 came out.  I'm certainly less than happy with my current service, whenever my roommate plays games on xbox the same time I am playing on my PC we both have tremendous lag.  Even streaming netflix seems to suffer at times.  I have a SB6141 DOCSIS 3.0 cable modem.  I've had it for over a year now with little to no issue before this so I do not believe it to be my modem.  I also have a direct connection to my router from my PC, and as I said this issue exist on my roommates XBOX as well so I know it is not simply my PC.  I would have to believe it is a configuration issue on Cox's side however I am open to other suggestions as well.