The start of the 6:10AM PST Bambino Fondo yesterday was a bit of a disaster. My graphics card, which rarely drops below 57 fps, struggled at 24 fps. (And in case @Paul_Allen is reading, every rider was wearing the same kit!) From the starting pen to the first intersection, riders kept spontaneously popping into and out of existence around me. After the ride, Zwiftalizer showed a huge peak in network errors at the start of the event.
It seems that 1000+ riders in the same place at the same time is currently too much for the game to handle with aplomb.