Not only does it depend on the speed of the flash drive, but also how much
RAM you currently have on your PC. Using the exact same 4 GB flash drive, I
used Readyboost with 2 GB and 4GB RAM on my PC. There was no improvement
with the 4 GB installed, but quite an improvement with only 2 GB.
(Readyboost does accomplish not only the Page File usage, but also all disk