Stumbled upon this thread and figured was a good enough time to finally register to post after a few years. Noticed the same issue as you and others around the same time mostly with Steam but can't say about the others as of those listed I only use Google otherwise. Web wise and through their front end it's been sluggish to non-responsive but on downloads everything is full speed. Doubt it coincides with their year end sale as it wasn't a issue during prior events. Usually see cdn2.* throttling for a few seconds instead of just loading as it should do. Figured it was just my AV going wonky lately so nice to see it wasn't that.
I only noticed the issue after I swapped out gateway units back on the 9th or 10th as my old 2700B finally died after 7 years. I wasn't going to wait on a replacement ps so just swapped in one of the newer 4111N's off the shelf that weekend and that's about the only site I've noticed issues with so far from your list. Although one night on a heavy download on Steam it reset the unit until I adjusted their line speed setting. Just chalked it up to using the unit as a router now unlike previously when I used the 2700 as a bridge to a router.