We have been struggling through the last few weeks of snapshots with huge CPU load crippling our main server (dedicated box, quad core Xeon, SSD raid, Gig connection). Running system monitoring tools from the command line we could see Java usage bouncing up to 200%.

Repeated the tests on dev boxes, fresh worlds, removed all CB's, redstone, no mobs etc. More testing has shown that a new world is fine on CPU load with only one player on in the end ( 4% java cpu load ) but overworld & nether between 50 and 200% cpu load. However in the nether if one gets rid of lava source blocks causing falling an flowing lava, and waits for a while the CPU load drops to around 9%.

Moving into fresh chunks with lava flows causes the CPU usage to climb again.

We concluded that lava flows are causing huge problems for the game. This may extend to water too, but lava certainly a huge problem.