I installed the demo of Aperture 3 and was pretty disappointed with it. It was just as slow as iPhoto. My setup uses networked (GBe) home directory, ~25,000 photos.
I just did the update above...
it took all day practically to rebuild library but now...
MUCH BETTER!
Aperture works great now in terms of speed when navigating photos.
I believe that 99% of the angst about moving to AP3 concerns the new database structure needed for Faces, Places, and hopefully, Time Window integration. Probably only takes 2-5 seconds to update each image, but that is a long time if you have 10+k images. Longer if you elect to reprocess all of your masters. Not an issue if you simply start with AP3 or if you create a new AP3 library and import all of your AP2 projects one at a time. The final issue is that conversion of an AP2 library is a bear if you don't have lots of free disk space. Quite by accident, I did this right as I copied my 150 GB AP2 library to a 1 TB disk, converted it to AP3, and then copied it back. Took forever to do all of this copying, but I had no crashes.
If you are comfortable using multiple drives and managing a lot (well over 10k) of images, then consider this:
-- The Aperture Library is a data base. It contains several different items, all of which are processed differently.
-- Master image files are static. They are written to disk and there after never written again; they are read only.
-- Then there are the "Versions." They are read every time look at an image to adjust it and written every time you actually move a slider. Then there are those pesky thumbnails and previews (which are always reprocessing) which are what you see when you browse the Library. (And possibly what you actually see when you edit at less than full res.)
-- Finally, there are all of the indices of the library structure (folders, albums, projects) that hold the whole mess together.
It follows then that master image files can be written out to a large, slow disk (even USB will work) as they are read only rarely and never rewritten. The rest of the Library, on the other hand, needs to be on a fast drive. And, in extreme cases, this drive may need to be defragmented regularly. (The "Rebuild" function may do this; I don't know.)
This argues for the following structure for max speed:
-- Aperture application: On the fastest reading drive you can find. Consider one of the new SSD. (Writes are slow, but you never write to the Aperture Application except when you do a software update.) But remember, after the initial opening of the application, you don't read the disk all that much - the whole application is only about 700 MB and most Macs are running at least 2 GB of RAM (laptop), 4 GB for iMacs, and MacPros should have 8 GB. So while I have read a few peans to the idea of using a SSD as your boot disk, I'm not sure it is really worth it.
-- Aperture Library: On your fastest drive. Ideally connected via eSATA or at least Firewire 800. You may want to limit what else is on this drive so that you can copy it out, reformat, and copy all of the data back every quarter or month. This is the surest, safest and best way to defragment the drive. While all of your editing may be done in RAM, this disk IS being written all of the time.
-- Aperture Images: Set up as "Referenced" Image on the previously mentioned slower drive. No reason not to use a fast drive, but you really won't see any difference as you are only writing to this disk the first time you dump the images to disk, and only reading when you edit at full resolution, print, or when you export.
Obviously, if you use this set up you must remember that Vaults will NOT back up your actual image files so you must use Time Machine and/or SuperDuper! or similar to assure that the images are backed up.
This kind of structure is easy to set up inside a MacPro but can be done with any Mac by using external drives. There might be some advantage to using a RAID for the Library, but I doubt that it is worth the effort/cost.
So you might end up with:
-- Drive 1: High speed drive. OS, Apperture application. (Might be the one that came with your Mac.)
-- Drive 2: Killer high speed/high tech drive. High RPM, big buffer, "enterprise" grade, etc., etc. Speed is more important than size. This is where you put the Aperture Library.
-- Drive 3: Nice dumb, reliable drive. Image files in a simple Images/Year/Month/Day folder structure. (Might be a place to reuse your factory drive, especially if it is older and slower and you want an excuse to buy that hot new drive for your boot disk.) Alternatively, you could use that HUGE USB 2.0 pig that your PC owning friend is dumping as it is to slow to play Quake.
With a MacPro, this could all be done internally.
Offered with no guarantees, but based on a few years experience with ADP, Macs, and a careful read of some of the better posts on the Internet. (Those of you who grew up with Mainframes and Minis will find some of this familiar.

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A N D , A S A L W A Y S : Have a serious backup strategy and use it. With Time Machine, at least, you have no excuse!