While I haven't used the Google service yet, I see similar problems in a lot of public areas like airports where I happen to find myself a lot. It seems to be more of an issue with the non-direct data traffic like the auth services, ads/gateway tasks, and DNS. More often than not it is one of these 'services' that are unrelated to the traffic that are acting up.
One example is the wi-fi networks in the Minneapolis or San Fran airports. You can log on, and then getting an IP, getting on the "I agree" screens, the videos you have to watch etc etc are all dog slow to one degree or other. The Delta lounge in the Minneapolis and San Fran airports are very extreme examples of this problem especially when they were T-Mobile (damn their black souls). You would 'get on' and then nothing or something trivial really slow.
Once on you would have decent ping times and some speed tests would be OK but anything that needed 'extra services' was pain. Changing your DNS to something you have or a know fast provider helped a lot which tells me the NAS/Radius/whatever server they use was overwhelmed. Now that I am thinking about it I should do a traceroute next time I am on to see what is happening in more detail, I am curious.
My first bet is that the majority of these services go through a single auth/security box that is under-CPUd and forces everything out a single overloaded link. If anyone has the time. I also wouldn't be surprised if DPI had a hand in it too, especially from Google.