I haven't used this extension but my guess is that it's defeating the host redirection that ISPs do to point you at their extremely shitty and underprovisioned edge caches. Most ISPs run them to cache popular youtube content and save on out of network bandwidth, however as with most things they are terrible at it and extremely cheap. The solution I've been most happy with is to run all traffic over VPN to an endpoint not under Comcast - keeps everything nice and speedy, albeit with slightly more latency.