ReadyBoost uses flash memory devices as a disk cache which can help “service random disk reads with performance that is typically 80-100 times faster than random reads from traditional hard drives.” You might think that if you aren’t connecting a flash drive, you don’t need this service, but ReadyBoost is also used to facilitate SuperFetch, which preloads applications based on past utilization to reduce load time. Once your PC gets to know you, disabling this service may actually make your PC feel slower, rather than faster, but the service is not required, and can safely be disabled if you so choose.