The Big Pit-o-Lag is a common phenomenonthat occurs whenever loads of stupid people hurry to a medium to small area. The game engine was not meant to play God in this way, so as a result, your computer slows things down a bit, or causes you to 'jump' a bit forwards in time. It may even stop you for five minutes.
Stupid people and normal people will insist on clustering into an area about the size of the bottom floor of the Lion's Pride in or larger. The poor game engine will then have brown-trousers time trying to get all of the polygons to move the way the engine has been programmed to move them, so it slows everything down to a manageable pace. Then it sorts out whatever the hell you were doing with the relevant animations while you wonder about why everything seems so jerky compared to before.
Stupid people moving away in high enough numbers will cause the Big Pit-o-lag to go away, but if they return they will cause it to lag again. And the sudden displacement of Morons would possibly cause another Big Pit-o-Lag to form elsewhere. In other words, if every person moved to somewhere else, then the Big Pit-o-Lag will go too.
Any areas which you feel was missed, feel free to add.