What do you mean by greater circle? The circumference of the earth is 40,075,000 m. Speed of light is 299,792,458 ms. So we're talking about 40,075,000 / 299,792,458 = 0.1337 -> 134ms to get around the earth once. As the earth is an ellipsoid that is mostly perfectly round, you can assume, that from any point A to B you'll just need to travel half of that, so we're at 67ms for light speed.

Looking at the 150ms now, which is about 2.3 times the 67ms, this kinda seems reasonable.

And yes, this is global purging - I just reconfirmed it with someone from Fastly.