CPU load periodically goes to full around a certain area.

In the last couple of days I have experienced severe frame rate lag and the CPU usage of Minecraft spikes to 100% for periods of time (followed by brief periods of smooth usage before spiking again). It occurred on the online server I play on, though I grabbed a copy of the world from the owner and the same issue occurs on a local server. I have tried with and without bukkit. I have tried with the 1.7,2 server and client I was normally using but also 1.7.4 and the latest w07 snapshot.

I use no mods etc.

My computer is able to handle minecraft easily and indeed even around this farm when I log in it's a nice smooth experience until it suddenly seizes up.

A copy of the world can be found at: https://dl.dropboxusercontent.com/u/37480600/seqkg-vanilla-2.zip

The farm itself is actually disabled at present (I removed a torch from the item elevator and the spawning area is lit up with a switch), but the problem persists.

Leaving the area of the farm and I experience no issues.

This has pretty much made this area of the world a no-go area and I really don't understand why.