Since you use a flash USB of recent models of 1 GB or more, connected to a USB 2.0 port, you really notice any difference in loading time of programs. Many games and applications can achieve higher load in half the time, besides the overall performance a little better (especially on systems with only 512 MB of RAM). But if you measure the transfer rates of USB drive and HD, you'll realize that almost always, the HD is faster. How then can the flash drive to improve performance?
The central issue is that HD is fast on sequential reads, which reads large blocks of data, located in adjacent sectors. The HD has a very high access time and therefore can offer incredibly low transfer rates (often 2 MB / s or less) reading many small files scattered. In this aspect the flash memory takes a great advantage. To give you an idea of the difference with an HD access time of 13 milliseconds would be able to accomplish little more than 60 readings per second, while even a modest flash memory speed can easily perform more than 4,000 readings per second.
Another issue is that the flash drive and HD are two separate devices, connected to separate buses, so the system can read data in both simultaneously. The system then copies small files and small sectors recorded far apart from the HD to the flash memory, and use it to store some swap memory (example of application where takes advantage of low latency of the flash memory) then the HD can concentrate on reading large files, depending on which is faster.
Flash memory is non volatile, so data is still there, ready to be used in subsequent boots, without needing to be transferred again from the HD. The main problem is that ReadyBoost flash memory has a limit of read cycles, so that heavy use can cause the flash memory to be defective after one or two years of use, especially the cheaper flash drives, which use chips of lower quality.
With the popularity of ReadyBoost feature, manufacturers of flash memory began to launch various types of flash drives optimized for ReadyBoost. Initially high-performance models were built using chips and controllers capable of sustaining higher rates of transfer. Then began to be released "dual-channel" pendrives, where two Flash memory chips are accessed simultaneously, doubling the rate of reading and writing, very similar to what we get when using two drives in RAID 0.
Finally, there were internal-use of flash USB, again sold as specific models for ReadyBoost. These internal USB drives are installed directly into one of the USB headers on the motherboard, the same 9-pin connectors where you connect the front USB ports of the cabinet. The idea is that they are installed continuously, keeping the ReadyBoost cache.
Each USB header on the motherboard provides two USB ports. However, the two ports are connected to the same controller, so the share offer 480 megabits per USB 2.0 controller. Making the flash drive to occupy the space belonging to both gates, the manufacturer avoids the performance of the flash memory to be underused by large use by the connection of other USB device.