Since the SD card slot on the 2013 MacBook Air is connected via the USB 3 bus (instead of USB 2), it shouldn't be limited anymore by USB2 speeds (i.e. theoretical maximum of 480 Mbit/s). The new limit is 5 Gb/s.
So I ran a little test of whatever I've got lying around. I used the Black Magic disk speed test, with a 1 GB test file.
|SanDisk 2 GB micro-SD card
|Came with my Android phone
|Kingmax 16 GB micro-SD card
|Bought to replace the small SD card above
|Nikon-branded 2GB SD card
|Came with my Nikon point-and-shoot camera
|No-name 2GB SD
It's not a very interesting test; none of these is above the theoretical maximum of 60 MB/s for USB 2.