ESX shows excellent performance when using more than one physical CPU in combination with the "virtual CPU" feature and disabling HT.
"It becomes clear with the numbers in front of us that Virtual Server is in serious trouble compared to ESX 2.5. The main areas of concern are Memory Bandwidth and file system IO. For a single processor system the Memory Bandwidth was running at 34% and 33% relative to a bare metal installation for non hyper-threaded and hyper-threaded respectively. Similarly, the file system IO was at around 40% of the bare metal installation compared to ESX which hovered between 80% and 98% depending on the processor and hyper-threading combination."