LR2 64-bit slower than 32-bit (Mac)

Question

I ran a test a while back with Lightroom 1.3.1, generating 1:1 previews on a test directory of images:
http://www.rassoc.com/gregr/weblog/2008/02/24/mac-pro-performance/
On a Mac pro 2.8 GHz 8-core machine with 12GB of RAM, I found I could generate 211 1:1 previews for my directory in about 5:36, or 1.59 seconds/image.
I've found Lightroom 2, in 32-bit mode on the same machine, to run roughly the same speed.
However - running Lightroom 2 in 64-bit mode on this same machine takes 7:23, or 2.10 seconds per image. It's typically using >500% of CPU.
Why is it so much slower running 64-bit?

Answer 1

My results are repeatable; 1:1 preview generation takes about 30% longer using 64-bit than 32-bit. I opened a support ticket on this (case number 180331555) - here is the response from Adobe:
I understand that Lightroom# takes longer to render previews in 64-bit
mode than in 32-bit mode.
I am able to duplicate this basic behavior when rendering a relatively
small batch of previews in Lightroom.
It is not expected that all operations in 64-bit mode will perform
faster. Memory access generally takes longer in 64-bit mode due to
having to address a larger memory space.
Performance benefit should be most noticeable when performing operations
that use more than 4GB of memory. At this point, the 32-bit application
will experience a large performance penalty due to increased hard drive
access.
Hmm. Sounds like they were able to reproduce the behavior I see, but I can't think of why memory access should be slower in 64-bit.