That big of a difference in speed, by a factor of 4 or 6, feels like it's likely to be some quirky weak spot or instruction that the old chip is slow with.  Unless it's a touted feature of the i5 that they made SSE2 six times faster.

A quick summary:
Xeon Quad        41% slower
Core 2 Duo        55% slower
Core 2 Duo        same (vess)
Core 2 Quad      50% slower
Core i5            200% faster (nelisky)
Core i5            100% faster (vess)
AMD Opteron    105% faster

aceat64:
My system went from ~7100 to ~4200.
This particular system has dual Intel Xeon Quad-Core CPUs (E5335) @ 2.00GHz.

impossible7:
on an Intel Core 2 Duo T7300 running x86_64 linux it was 55% slower compared to the stock version (r121)

nelisky:
My Core2Quad (Q6600) slowed down 50%,
my i5 improved ~200%,

impossible7:
on an AMD Opteron 2374 HE running x86_64 linux I got a 105% improvement (!)