Speedtest consistently shows upstream speeds of approx 6Mbps ever since I switched to new cable modem Arris SVG2482AC (Docsis 3.0). The previous cable modem (XB6 aka CGM4140COM, also Docsis 3.0) usually reached around 39 Mbps upstream. I have gigabit service. Downstream speeds are fine, i.e. they range from 400 to 700 Mbps, similar to previous modem. Yes, I know that Comcast clearly states that they limit the downstream rate to 856Mbps. My problem is only with the upstream speeds. Note that the Comcast web page ( https://www.xfinity.com/
3 to 4 QAM 64 upstream channels are all that Comcast currently offers. They are more than enough to deliver the advertised upload speed of 35 Mbps for the Gigabit speed tier. Something else / some type of upstream impairment is going on.