Aperture is a rather complex application and image files are getting larger. All of this can lead to slow downs and other problems. And this, in turn can lead to a bit of thrashing by folks who don't fully understand how Aperture (and Lightroom) works.
Aperture is a data base application. An Aperture application starts out as two files/folders:
-- The Aperture application, normally installed in your Applications folder. This folder, of just under 900 MB, is a "Package" full of goodies that actually runs the application. It belongs on your fastest drive so that it can load into RAM when you start the application. No mysteries here.
-- The Aperture "Library." This is another Package, that is, a folder that does not normally open. Open it, by right clicking and you will see a list of folders. The most interesting, the only ones that you can really control, are labled "Previews" and "Masters." The largest single item in the Library will be your Masters, followed, normally, by your Previews and, finally, your Thumbnails.
So how can you make your Library smaller?
-- Take fewer photos.
-- Shoot lower resolution Masters, ie., JPG as opposed to RAW.
-- Reduce the size/resolution of your Previews.
When you are using Aperture, you spend most of your time looking at Previews and Thumbs, not Masters. In fact, the only time that I am sure that you are actually using the Master is when you export, print, and when you view at full resolution. (Others may be able to clarify this.)
So I find the following to be reasonable:
-- Set the size of Previews at or one size smaller than the resolution of your largest monitor.
-- Set the quality as low as you can accept, generally 6 - 8.
-- Rebuild your old Previews and Thumbs at a lower size/resolution. There was a problem with one release of Aperture 3 that caused it to create bloated Previews and Thumbs. If this happened to you, then deleting your Previews and Thumbs will get you back some space. (Be sure you backup first and have ample time and disk space.)
That is about it. You can make slight reductions by limiting your metadata, or not using Faces (Faces create tiny thumbnails) but none of these amounts to much.
So what's next? RAM,RAM, and more RAM! Why. Because during any given Aperture session it is going to read the application, scroll the Thumbs, read the Preview, read the Versions, and, finally, read the Master. And then it will rewrite various of these as you adjust. If everything is in RAM, this will be fast. If not, then you will have to page to disk and this will be slower. And if your hard disk (HD) is full, much slower.
ALL HD slow as they fill. So, for best speed, you want to keep your HD as empty as possible, the sweet spot being somwhere between 50% and 75%. (Bigger HD can get fuller.) Why? Because Mac OSX is constantly making work files and rearranging data for the best fit and speed. The more free space available for this, the better.
So, after you buy all of that RAM, spring for the multi-TB HD.