After Ruud Lathrop, Thomas Conover, Seronis Zagato and others tried to bring attention to the grid performance problems on Monday (Zagato, notably was reportedly threatened with a banning by a Linden Lab liaison for her efforts to inform affected residents), Linden Lab have found the issue and are today progressively restarting the grid with updated software that they believe will fix the issue.
The restart should be complete around 1-2 PM SLT (US Pacific). This restart is running North to South, so if you get a warning that the simulator you are in is about to shut down, you can teleport several sims northwards to escape the restart wave.
[Update: 1pm SLT - The rolling restart is now complete]
