Transfer speeds are a tricky area and are determined by many factors, such as the connection, rather than just the speed of the device, but a basic rule of thumb is below:

USB 1.0 – 0.192 MB/s
USB 1.1 – 1.5 MB/s
CD-ROM – 7-8 MB/s
FireWire 100 – 12 MB/s
100Mb/s Ethernet – 12.5 MB/s
SD Card – 6-30 MB/s (minimum of 6MB/s, maximum depends on card)
FireWire 400 – 50 MB/s
USB 2.0 – 60 MB/s
EIDE HD – 80 MB/s
GIGABIT Ethernet – 125 MB/s
SATA 150 – 150 MB/s
SATA 2 – 300 MB/s

Things like USB hard drives, and thumb drives would depend on the USB connection, i.e. USB 1.0 or USB 2.0. But unless you are transferring large files you aren’t like to notice the difference between USB2.0 and SD cards, many people, including myself, use SD cards as hard drives and don’t notice any difference in speed in day to day usage.

As far as MP3 go, even at the maximum bitrate (320 kbit/s), it is still only 0.04 MB/s a second needed to play it from the card, so an SD card will handle MP3′s without any problems (most MP3 and MP3 enabled phones use some form of SD cards, such miniSD or microSD).

The only time you may notice a lag would be playing high definition movies, HD-DVD and Bluray both use about 4.5 MB/s bit rate, so you’d have to ensure you get a SD card that has a speed rating of Class 6 – which is 6 MB/s and above.