I found an article at Tom's Hardware explaining SuperFetch and Readyboost. We a have a lot of information about Readyboost so I found the part of this article dedicated to SuperFetch more interested.
SuperFetch tries to relocate application data from the slow hard drive into all available memory. It utilizes the available capacity to create a so-called warm memory state for the single purpose of making applications available almost instantaneously. However, SuperFetch needs a certain amount of main memory. At only 512 MB RAM size, the feature won't be very efficient, as Windows plus 2-3 applications will already eat up the total memory capacity. There won't be main memory space left to pre-cache application data. If you don't work with multiple applications at a time, 1 GB should be enough to see a positive impact of SuperFetch when compared to Windows XP. However, we experienced the best results at a main memory capacity of 2 GB - more won't hurt either.
The last portion of this quote explains why in devices with 512 MB and up to 1 GB we have seen so much impact after disabling the SuperFetch.