Found the following article on this.
While buying a new USB flash drive, the average customer usually cares only about the price, the look and the storage volume of the drive, but not the data retention (simple said, how many years you can use the drive), the quality of the memory chips and the read/write speed of the USB drive itself. 95% of the USB drives that are currently selling on the market are using the MLC (multi-level cell) technology, which is cheap, but the data retention rate of the NAND flash chips and the read/write speed is very low. The SLC (single-level cell) technology gives the data retention rate 10 times more than MLC (you can read about SLC vs MLC at the official page of Buffalo, one of the major USB drive manufactures; and here is another article on the life of SLC and MLC-based drives), and a significant higher read/write speed. To be exact, the average read speed of MLC-based USB drives is 10-15 MB/s, while SLC gives 20-25 MB/s, sometime to 30 MB/s. The write speed of MLC-based USB drives is even worse, around 8-12 MB/s, while SLC-based drives reach 20-25 MB/s. The only disadvantage of the SLC technology is that the production cost is really high, and after 2007, most of the manufactures of USB drives ceased the production of SLC-based USB drives (except some very hi-end series like Buffalo RUF-S or RUF-R series, Sandisk Ducati Extreme or Corsair Voyager GT).