Why does it take so much longer for applications to launch under lion as compared to snow leopard

Question

I have two iMacs one which cannot run Lion and one which does.
Launching applicaitons under Lion takes a measurable amount of time longer to launch under Lion than they do under Snow Leopard.
The Lion machine is newer, faster and larger than the Snow Leopard machine - 4 cores vs 2, however, every time I launch an application, such as Safari or Text Exit as two examples, the Lion machine just cranks and cranks (you can listen to the disk drive).
Text edit is particularly rediculous. If I try to open a text file, either for the first time or one which has been opened by text edit before, I am greeted with the sound of the disk drive and the spinning beach ball.
All hardware diagnostic tests pass with flying colors. Or so it appears as no errors or abnormalities are reported.
This effect has been in place since the lion machine arrived (which was "factory installed" with Snow Leopard and then upgraded over the net to Lion).

Answer 1

That is not normal, but what is going on is hard to say without some testing.  It could be cache-related, if you've gotten some corruption in some important caches.  Try getting a copy of OnyX and clearing caches.  (Note that cache clearing should be considered a troubleshooting step only...  don't use such utilities for routine maintenance.)
You could also have a dying hard drive.  That would not necessarily show up on hardware tests.  Sometimes drive failures show up in advance in hardware tests or SMART status checks, and sometimes they don't.  That could explain the excessive hard drive thrashing.  If this is the problem, there may not be any reasonable solution until the drive finally goes kaput, or decides to change to a detectable imminent failure.
There could also be any number of other problems, such as low RAM, hard drive directory corruption, incompatible third-party software, etc.  Try some of the techniques in the Mac OS X Speed FAQ.