Why is rendering "Multiple frames simultaneously" so slow when "Skip existing files" is enabled?

Hi,

I've tried rendering several comps (usually HD res, some 4k) with these settings both with networked computers and on just one computer and it always seems to take much longer to render.  Mostly, AE seems to stall when it tries to figure out what frames to skip (from what I gathered).  I've also tried many memory/processor settings using Adobe's recommendations below.

http://blogs.adobe.com/toddkopriva/2010/10/please-try-recommended-memo ry-settings-for-after-effects-cs5-and-give-feedback.html

Any idea why this may be?  We really wanted to take advantage of the many cores and RAM our machines have in our studio.  We're using Mac OS 10.6.4 mainly on Mac Pros and AE CS5.

Anyone have any good benchmarks for this?

Regards,

Jason