I've been playing around with USB thumb drives, SD cards, and SSD hard disks lately. Mostly because I've been interested in the read/write performances of the devices when implemented to do atypical tasks. (aka: boot an operating system, stream some media, etc.)
A few fun facts
USB 2.0 = Specification says around 480Mb/s. (~55MB/s) In reality, most drives are hovering around 30MB/s read and 20MB/s write.
SD Cards = This was interesting... I wasn't aware of this but SD cards have different "classes" that determine their speed. The classes are numberd (0,2,4,6,10) The rating system only shows the minimum required to be in that class. So a class 10 card has to be at least 10MB/s. Right now it looks like these devices top out around 20MB/s for high end class 10 cards.
SSD Hard drives = This gets really complicated since read/write performance directly relates to block size, sequential/non-sequential operations, etc. But... generally speaking, SSDs today are averaging around 200 MB/s write and 250MB/s read.