How exactly are you testing your connection? That's a relevant bit regarding networking. If testing in phoenix, test a LA California-based server, as most cox residential egresses there. I like Race Communications out of LA to test against on
speedtest.net. Anything else hits interstate transit, more than cox already does dumping you out LA peering points.
Your host nic can handle line rate, and so can the kernel in most cases, really the question is your transport and the applications using it.
If using a
speedtest.net sort of test, then ymmv with the server you're testing against.. If dealing with comcast, I don't know their peering infrastructure as much as cox's, but they're not typically local, and probably transport you to another state for egress. This isn't much different from cox, they dump you out LA or Dallas, usually the former than latter, only cbs and other direct peering relations egress actually in phoenix. Post a traceroute to 8.8.8.8 if you want some recommendations to test against for real world usage.
Look at your modem levels too. You can hit your moto/arris modem on
http://192.168.100.1 url to see your modem levels, this has existed since 2000 or so with the advent of docsis and motorola not sucking. SNR, RX/TX power are what you want to look at, and plenty of wikipedias about docsis to explain tolerances. If not within tolerance, call the provider to start digging and replace coax as necessary.
Every few years my cable gets crappy, and I have to call them to help me dig up and replace bits of either the yard or house to replace. Arizona is hell on coax with suck-out and other syndromes of extreme heat-based dysfunction. Expect your levels to go to hell every 3-4 years in arizona with coax.
HTH!
-mb