This is an interesting example of why randomization in experiments is important. If you allow users to self select into the experiment and control group, and then naively look at the results, the results might come up opposite from what is expected. This is known as Simpson's Paradox. In this case, it was only the users for whom page load was already the slowest that picked the faster version of the page. So naively looking at page load times made the pages look like they loaded slower.

However once Chris controlled for geography, he was able to find that there was a significant improvement.

Moral of the story: run randomized A/B tests, or be very careful when you are analyzing the results.