Since updating the world from Pre3 to Pre4, Everytime the server reboots, if you go to the nether, it hangs for ~10seconds. Then each place you go to it hangs again. But only the first time you go to that location from the nether.

Example:

Server Reboot

Home Base > Nether - hangs 10s

Nether > Home Base - No lag

Home Base > Nether - no lag

Nether > Other Base - Hangs 10s ect