EVE Online's truly massive single-server design that puts all players in the same universe is that lag has been a constant problem since the game's launch. For the most part, the game runs smoothly and lag doesn't affect the day-to-day lives of players. It's when too many players congregate in one place that the server node they're on begins to grind to a halt. Circumstances can rapidly go from bad to worse, with server nodes collapsing under the strain and database failures occurring.
It's been noticed by players that lag in fleet battles has gotten considerably worse since the Dominion expansion came out. To get to the bottom of the issue, CCP Tanis has been scheduling regular test-server events in an effort to replicate the conditions experienced on the main EVE server. By logging a fleet battle in action, CCP hope to find ways to resolve fleet lag and prevent further lag-causing defects from reaching the main server unnoticed. On Saturday 29th May between 20:00 and 21:00 EVE time, CCP will be running an important hour-long testing session. They're asking players to turn up and help out, as they need at least 300 pilots to begin testing and ideally would want over 500. For information on what the testing consists of and how to access the EVE test-server, visit the official mass testing sticky thread on the EVE forums.