Rio’s Olympic pool is certainly not slow. Nine world or Olympic records have already fallen, and fallen again, in Brazil, and the swimming competition at the Games is still only halfway done. But is it fast? Does the credit for those record-breaking performances lie with the Olympic athletes’ dedication, or the Olympic architects’ design?

Beijing’s Olympic pool was acclaimed for its speed eight years ago. “It’s by far the fastest pool in the world,” said Rowdy Gaines to NPR in August 2008. Gaines won three gold medals for Team USA in the pool at Los Angeles 1984, and now works as NBC’s swimming analyst. A total of 29 records were set in China. Four years later, at London 2012, just 19 new marks were set. Rio is currently on course to tie London, but with only 16 events left, cannot catch Beijing.

In London, 11 of the winning times were slower than in Beijing. Of the 16 gold medal times registered so far in Brazil, five are still slower than those from eight years back, despite any advances in sports science and coaching techniques that may have come since then. (The biggest improvements have come on the women’s side of the competition, where all of the times in Brazil have so far beaten those in China.)