Yeah, be sure to try each test multiple times. You can get that much of a variance simply from the garbage collector running at one time but another, from the file needing to be read from disk vs. already cached in memory by the operating system, some other process hitting the CPU, or in EC2, the virtual CPU getting fewer cycles at the moment...
Seems to work. (require (file "lib/.../foo.ss")) appears to be the right thing to do for .ss files; a Scheme load uses Arc's readtable which messes up on square brackets, and a plain (require "lib/.../foo.ss") doesn't like periods in directory names.