My download speed on my home wifi is normally 5mbps. When I connect to google based sites, it drops to 10kbps. At first I blamed my computer, but then I tried three different speed tests. Xfinity told me I had 20mbps which is obviously a lie, google's showed the 10kbps, and the reliable test from ookla showed I still had 5mbps. I was able to swap between three different tabs to see three different speeds which then made me remember net neutrality was killed so I started looking into that. I tried several public xfinity based wifis and still had trouble with google. Connected to a wifi that was not xfinity, google suddenly worked just fine. Next thing I do is I find the submit feedback button to send xfinity a nasty review when I find that the feedback button is broken. Instead of sending me to a proper page, it simply takes me to a jpg that is sized 1 pixel x 1 pixel. After all of this struggle, I can't help but feel like xfinity is purposly antagonizing customers. If it turns out I'm wrong, then I ask you to prove me wrong because I want to believe this is fixable and not just a nightmare.
Comcast does not throttle. Your issue lies elsewhere.
I had same problem. I called google support and they said I they need the mac address, so they could have xfinity allow it. I hung up......and as far as throttling, when I am on the phone with xfinity, i have wifi speeds of 70mbs. Later it drops to 20-25....is this a cooncidence?
Run a traceroute or a ping test to the "google based sites" (since you didn't specify what they were) and see what you come up with.
Since the speed tests show that you're getting the speed you were expecting, the issue isn't on Comcast's end. As pointed out earlier, Comcast doesn't throttle your speed.
As far as inconsistent WiFi speeds, that's kind of the nature of the beast as it's subject to all sorts of interference. If you want consistent download speeds and connectivity, use Ethernet.