How can I determine what is a normal USB transfer speed for my Android phone?
There's no such thing as normal USB transfer speed. It depends on several factors including but not limited to:


Class of the sdcard 
USB speed (1.1 vs 2.0)
Performance and load of the the computer 


Out of these three the one with the most influence is the class of the sdcard. The class describes the minimum write speed of the card in MB/s. For example a Class 10 card has a minimum writing speed of 10MB/s.
Based on this your sdcard is most likely a Class 2.



If you can't locate your sdcard Class, there's a free tool (only for windows users) that gives you the transfer rate of your sdcard:

H2testw 1.4


  After you've finished verifying your unit you will be able to see the exact time it took for the operation to complete, the number of megabytes tested, and, of course, the writing / reading speed.




As a side note: The cache that Abdul mentions has nothing to do with this. That's for reading and only applies if the card is mounted in the phone.