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.
| Card | Read speed | Write speed | Remarks |
| SanDisk 2 GB micro-SD card | 17.4 MB/s | 3.4 MB/s | Came with my Android phone |
| Kingmax 16 GB micro-SD card | 21.0 MB/s | 9.3 MB/s | Bought to replace the small SD card above |
| Nikon-branded 2GB SD card | 11.3 MB/s | 5.6 MB/s | Came with my Nikon point-and-shoot camera |
| No-name 2GB SD | 13.7 MB/s | 5.2 MB/s |
It's not a very interesting test; none of these is above the theoretical maximum of 60 MB/s for USB 2.