Similar issue. The speedtest.xfinity.com result is barely more than 50% of the paid-for speed. Mid-Feb, I exchanged the Motorola SB6120 w/ Xfinity SB6 at the local office. Issue remained. Upgraded to 300mbps plan. Issue remained (i.e. speedtest.xfinity.com result was barely over 150mbps - but quickly falls off as time wanes). Upgraded to 600mbps plan. Issue remained (i.e. speedtest.xfinity.com result was barely over 300mbps - but quickly falls off as time wanes). A couple weeks ago, Comcast brought out a replacement SB6. Issue remains. Within the hour, speedtest.xfinity.com result is in the 200mbps range. Within the hour following that, speedtest.xfinity.com result is in the 100mbps range. And drops to lower than 100mbps (ex: 60-80mbps) after that. On a 600mbps plan! Test configuration: New/unused Comcast-provided coax ran from Comcast's amplifier (at back of house) to back of new Comcast-provided SB6. New/unused Comcast-provided ethernet cable ran from back of new Comcast-provided SB6 to back of high-end laptop. Rebooted both the SB6 and laptop immediately before test. Note: Getting same result inside the house, regardless of which computer is hardwired to the SB6. Note: Speed degradation is considerably more during mid-afternoon and evenings. Note: Earlier this month, Comcast checked everything at the back of the house and said everything checked-out fine. Another tech will be coming in the next few days to check on things again. Maybe they should run a new line from the pole, and maybe replace the amplifier?
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