If anyone is interested I ran several tests on the sd card slot. It's not a slouch by any means. I consistently get speeds of 40 mbps write and 60 mbps read. I can run this test because I'm running off a hard drive and only using the sd slot for initial boot. I'm using a Silicon Power 16GB MicroSDHC UHS-1. I've attached a copy of one test using Gnome-disk-utility. Pretty darned fast and actually faster the the performance from the usb 2.0 ports.
Nice reading thanks. What I have noticed is that although the SD card performs faster in a simple test the USB 2.0 hard drive seems to preform better in real world usage. That helps explains why it's so :-)