Mach

"Quite simply, a monolithic kernel like the one used in Linux or most of the other Open Source Unix clones is inherently two to three times faster for integer calculations than the Mach microkernel presently used in OS X 10.4. That's why the world hasn't embraced xServes, for example, because for simple web or database service they are slower and serve fewer users.

Good God, people. Stop. Just stop.