UPDATE: nope, didn't work for long.
Hi everyone. I just now signed up on Android Central to post a solution I found. (apologies if this was already posted earlier, this is a long thread!)
I was having this same issue and after digging through forums from Comcast, I think I found an easy fix:
I took out special characters in my SSID (password) for my 5Gz wifi network.
The SSID had an @ in it and after changing it to letters (upper and lower) and numbers only, the slow internet connection on my phone resolved (I will update this post if that changes). I had to "forget" the network on my Samsung S5 and login again with the new SSID. This restored the normal wifi speed for all apps.
Overview:
I would try to load web pages, youtube videos, facebook, LinkedIn, Pokemon Go, and other apps over my home wifi connection and the pages would take a long time to load; sometimes, they never loaded. This had been happening for weeks (ever since Jan 2017 I think) but never realized that it was only happening on my home wifi. If I turned off wifi and let the phone connect to 4G, the internet worked just fine. I am burning through my data plan and suspect this was the reason and not Pokemon Go, which was my original suspect.
Wiped App Cache -- Cleared the cache of all the troubled apps. Problem remained
Cleared the Cache Partition -- Cleared the system cache for the phone itself. Problem remained.
Airplane Mode and Wifi On -- this worked. Turning on airplane mode and then turning WiFi back on solved the problem, but I wouldn't receive calls so this wasn't a realistic fix.
IPv6 -- I am unable to disable this on my Arris modem from Comcast (I think anyway. There isn't an option to disable it that I can find).
Software and Firmware updates -- I checked both the phone and the modem and everything was running the latest update (see below).
To change the SSID on the modem, I typed in 10.0.0.1 in a browser and logged in with the default username and password (it was just "admin" and "password") and when I did so it prompted me to change the password (good idea). From there, changing the SSID was under Connection --> Status. This popped up the "Local IP Network" and the 5Gz and 2.5Ghz networks. Clicking "Edit" on the "Private Wifi Network-..." gave me the option to change the SSID. Typed in the new one, clicked save, and was promptly kicked off wifi (of course). Once I logged in to the modem on my computer using the new password (copy and paste it!!!), all was good. I then "Forgot" the network on the Samsung and logged in using the new SSID. Slow wifi solved.
Device Details:
PHONE
Samsung S5
Android version 6.0.1
Last update was Jan 8, 2017
Wireless carrier is Verizon
HOME NETWORK
Comcast
Modem is an Arris TG1682G
Using the 5Ghz wifi