This mostly depends on when the drive was released and what memory was being used from which manufacturer, but currently the fastest USB 2.0 flash drives on the market are able to reach read and write speeds of up to 34 and 28 Megabytes per second respectively. Typically the manufacturer-listed flash drive speeds only apply to transfers of large contiguous files, whereas smaller files less than 1MB in size can dramatically slow down transfer rates due to the overhead for each file. Actual speeds are also largely dependent on the USB controller of your motherboard and how many USB devices are simultaneously plugged in.
Older high performance drives up to 16GB in size often use what's known as Single-Level Cell (SLC) memory instead of the more-common Multi-Level Cell (MLC) memory, thereby boosting write speeds for smaller files while simultaneously boosting a flash drive's endurance level. SLC memory is more expensive to implement however and offers less storage capacity for the die area, and over time has lost significant interest from semiconductor fabs like Samsung as consumer demand for larger flash drives increases. To make up for the loss of SLC memory, flash drive manufacturers have implemented a number of tricks including the use of quad-channel dual controllers and to a lesser extent custom drivers.
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