Well first bypass the router and go direct to modem.
Then run a test on Cox's backbone. Reply with what state you live in, and I will reply with the closest one.
If this is still slow, try running your computer in safe mode with networking, and running the test again. If still slow, contact Cox and have them look at your modem and see if they have any suggestions. If fast, and then it slows down when connecting the router, reply as such and then I will make my suggestions.