I don't know Redis' design, but all sorts of things can slow down a program's reading from memory, from CPU cache misses to mutexes, garbage collection, CPU utilization, or something else (I don't know Redis' design). Whereas the page cache and disk cache can be very efficient, without anything but more reads/writes to get in your way. Or maybe the program reading from memory is single-threaded, and the program reading from disk isn't.