Select the Disk Activity tab in Activity Monitor (Utilities folder) and click the Data button below the window. Copy a large file to and from the USB drive and watch the write and read speeds.
I have been frustrated with the very slow speed of reading and writing to flash drives, especially when my PC friends seem to transfer files many times faster. You know, a friend gives you some files or photos on a USB drive to load and you are embarrassed with the load time.
I found the problem, or more accurately, the feature that causes the slow transfer times. Your OS is indexing all the files on the memory stick so it can be easily searched. Turn off this feature and files are transfered very fast.
Go to:
System Preferences -> Spotlight -> Privacy
Add the name of the flash drive to this list.