USBDeview make test by writing a large file into the USB flash drive, and then reading back the same file to getting the read speed. The speed is calculated and displayed in Megabytes per second (MB/Sec).
According to Nirsoft, this speed test is made with sequential read and write operations. When using multiple small files, the read/write performances are usually much lower than sequential read/write.
Download USBDeview : http://www.nirsoft.net/utils/usbdeview.zip