There\'s not much to say about them. All I know is that certain areas of the level may cause players passing through them to steadily lose framerate untill they\'re only seeing 1 new frame every minute or so. There\'s many theoretical causes of this, but nothing has (or can be, really) proven.
Usually the best way to fix it is to type /spawn in the chat box and wait a few minutes for it to register, or hold the Foward key to run your character forward and, again, wait untill you can see the result.
I have heard of rare cases where, no matter what they may try, a character is never able to escape it, even after weeks of inactivity. They eventually had to create a new account.
