They use code _from_ a Microkernel, but that's about it.. It's sort of like a mash-up of Mach and a BSD kernel (FreeBSD, I think) with Mach-derived controlling part of the kernel's task and message-passes it to the BSD-derived side for other operations.
At best it can be described as a 'Hybrid Microkernel' since it does do some message passing stuff. Another famous 'hybrid microkernel' would be Microsoft's NT kernel. It started early in life as a microkernel design.
The kernel design may contribute to it, but there are a ton of little things about OS X that tell me that Apple isn't really concerned about performance very much. They create a slick interface and make things 'fast enough', which is perfectly fine for what the OS is designed for.