From what I understand, USB flash drives and solid-state drives (SSDs) are based on similar technologies, NAND flash memory.
But, USB flash drives are usually quite slow, with a read and write speed of 10-25 MB per second, while SSDs are usually very fast, about 200-600 MB per second.
Why are SSDs so much faster than USB flash drives? And why aren't USB flash drives faster than 10-25 MB per second?
Is it simply that SSDs use parallel access to the NAND flash memory or are there other reasons?
Parallelism, and how the controller makes use of it is the main factor. There usually isn't room for the 8-16 discrete NAND chips you'd find in an SSD. The controllers in USB stick usually aren't nearly as complex either, to efficiently make use of available parallelism.
Other factors that matter are the quality of the flash. Many USB sticks use cheaper flash which you have to read and write slower in order to avoid errors. USB drives don't have lots of over-provisioning to help keep clean blocks around. And USB 2.0 is limited to around 35 MB/s. And many SSDs come with a big RAM cache.
The main reason for these differences is price. There are some SSDs in a USB stick form factor, like the LaCie FastKey.
Some factors I've seen:
Parallelism: SSD's use a lot of Flash devices and access them heavily in parallel, pipelining and interleaving them.
SLC vc MLC: Multi Level Cells store two bits in each cell, but the access time is longer and Single Level Cells, which store one bit per cell. Besides, SLCs sustain a lot more write/erase cycles than MLCs. All USB thumb flashdrives and SD cards are MLC because it's cheaper. Note that some 'consumer' SSD's, like Intel's X25-M, also use MLC, reserving SLC for 'enterprise' drives, like the X25-E.
Complex allocation algorithms. Not only for wear-leveling (but that's very important too), they also detach the erasing from the writing, so when you have to write, the drive already has several pre-erased cells ready to be written.
Interfaces: SATA2 and now SATA3 are a lot faster than USB. Not only on raw bitrate, but also have a much higher efficiency. You never reach 100% of USB's theoretical speed, but on SATA, SAS, and IEE1394 you get it consistently.
Some extra hardware tricks; like a small battery-backed writeback cache. You write to this small RAM, and the drive itself writes to Flash after acknowledging the write.
The USB interface is a huge bottleneck. I happen to own a really cool Kanguru eFlash drive and it makes a huge difference.
This device has a USB and a eSata connector. It transfers at 45 MB/s on the USB side and 90 MB/s on the eSata side. From the same device! This truly points to that USB is the limiting factor. As others have said though, some drives a built too cheaply to even be limited by USB.
USB sticks also cost quite a bit less than SSDs. Which is because they use cheaper slow Flash instead of the expensive fast Flash used in SSDs.
The most significant constraint is probably that of size. USB flash drives need to be reasonably sized, so manufacturers can't exactly pack too many NAND chips into the drive. SSDs usually use the 2.5-inch hard drive form factor, which provides ample space for NAND. (Even the mSATA form factor provides more space than is available in a reasonably-sized flash drive.) As a general rule, the more NAND chips there are in a drive, the faster it can be, because the controller can write to more chips at the same time (in parallel).
Samsung gets around this problem altogether by making a "portable SSD" with full-fat SSD technology, with the same premium NAND and high-performance controller found on a "real" internal SSD. These drives connect to the computer via a USB cable rather than directly into a USB port.
There are flash drives in the traditional stick form factor that use full SSD technology. However, most consumers do not need or expect full SSD performance from a flash drive, limiting them to an expensive niche. Such drives are usually also large enough to block adjacent USB ports on most systems.
To reduce cost, USB flash drives typically use lower-grade NAND that isn't as fast as the chips used in SSDs, as well as less powerful controllers. Also, because flash drives generally have lower capacity than SSDs, there's less NAND relative to the other parts like the drive casing, circuit board, and controller which tends to increase the cost per GB as well.
Furthermore, the USB protocol has relatively high overhead. To attain maximum performance, the drive needs to support UASP, which lets the system send SCSI commands to USB drives. Less expensive drives typically support only Bulk-Only Transport which further limits performance. See this answer for more information.
Primary difference is the use of technology: SLC (Single Cell), MLC (Multi Cell), TLC (Three-level Cell). While SLC is the reliable and the fastest, it is also limited to maximum capacity in GB sizes on each Flash Chip. Slower Flash Drives that you are mentioning use MLC and therefore are slower, whereas majority of SSD are still using SLC.
