as has been stated in the Lightroom forums over and over again -
Lightroom has not been optimized for speed - at all.
Lightroom will be faster when v 1 hits the shelves.
(How much faster is hard to tell though).
Some people like to disable the "Build previews in background"
option found in the General tab in LR preferences. If you do this
LR will build the preview for a file on the moment you zoom in on a
image.
It will take a bit longer to access a specific image but you will
not have the initial slow down after import.
(It's also this initial preview generating background task that in
the current beta 3 slows down the app considerably - if you let the
task finish you will ha a speedy LR again).
And Beta 4 is near release… with some new features like "vibrance"
and "fill light" and more…
Check the LR forums for more info…
http://labs.adobe.com/technologies/lightroom/
Andreas
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http://www.pixels.nu