The system peaked at around 2,800 players fighting simultaneously, just 300 short of beating 2010's world record battle in LXQ2-T. Lag set in with around 700 players in the system, but EVE's Time Dilation feature kept the server running amicably up into the thousands of players. At the peak of the battle, the system was running at just 10% normal speed and people were experiencing several-minute delays on module activations. Dozens of valuable capital ships and supercapitals were destroyed, in addition to several titans worth several thousand dollars each. A few players filmed parts of the action and have shared footage of the slow-motion battle on YouTube.
[Thanks to BobFromMarketing for the tip!]