I saw the following nugget of information on the Kernel Newbies website:

‘Atime’ is the ‘Access time’ field of a file: When a process reads a file, its atime is updated. Disabling atime updates, with the ‘noatime’ mount flag, is probably the most used performance tweak that linux administrators use: An active server is continually reading files, generating lots of atime updates, which translate to metadata updates that the filesystem must write to disk. And writing those updates can seriously damage your performance. Believe it or not, a busy server like kernel.org (vsftpd + apache workload) cut their load average in half just by mounting their filesystems with ‘noatime’.