There's a
thread about this from 2005 that
shows memory accesses to be faster on P3 CPUs when I586_CPU and I686_CPU are enabled in the kernel config, compared to just I686_CPU.
Doing a google search for
"matt dillon I586_CPU" brings up more hits.
It seems I585_CPU enables a few features that I686_CPU doesn't, even on CPUs newer than the Pentium.