Stole this from Macaddict.com

Yes, we noticed this right before y'all did, and spent last night
and today figuring it out. It turns out the difference is
the fortran compiler (the inner, scientific part is in Fortran). The
previous core was compiled with the Intel Fortram 4.5 compiler, and
the newer one with the Intel Fortran 5.0 compiler. It turns out that
Intel 5.0 is about 50% slower on Athlons and 10-20% slower for Pentium
II-III architectures than the Intel 4.5 is (didn't test Pentium 4's).
So we have gone back to the Fortran 4.5, and the revised core is
posted on the web site. Autodownload of the core is controlled by the
work unit type, so the revised new core won't automatically update at
this point - you would have to quite, delete the old Core_65.exe, and
then when F@H... starts, it will download the new core. A lot of
functionality was added, but nothing was touched in the code of that
the non-700 series WU's does, so it really is all the compiler. We
tested the Intel 6.0 compiler today, too - faster than 5.0, not as
fast as 4.5.