I too received an email from Xfinity telling me that they had increased internet speed to 30-50Mbps in my area of SE MIchigan. I had to upgrade my modem to a doc sis 3 but still could not achieve the speeds promised. Contacted Comcast and tech replaced drop cable from pole to house and told me I needed a faster router and to trip branches away from drop cable. Bought a new Netgear 300+ Mbps dual band router - trimmed any interfering branches. Still only getting 11-14Mbps. Comcast chat reps were not able to increase speed by tweeking the modem and powercycling it several times. I was disappointed and frustrated. I searched Google for causes of slow speeds and came across an article that suggested that the firewall and/r the antivirus program could be the culprit(s). The firewall shutdown did nothing but when I turned off Avast free antivirus my speeds increased into the 30 to 48 Mbps range! I mention this for others that are not getting the speed promised.
Now I have to try to find an antivirus program that will not suppress the bandwidth speed.
Frank