How much RAM do you have, and how large are your original image files?
Do you have really large raw files, 60 megapixel or more?
If you are using Aperture 3 sensibly - not running too many Applications concurrently that also need much RAM and don't work with gigantic scans - Aperture 3 will be o.k.. You should at least have 4GB RAM.
Aperture 3 will make use of the available RAM to speed up the processing. My oldest MBP only has 4GB RAM, but Aperture works very well, as long as do not open iMovie, GarageBand, and Safari concurrently.
All cases I have seen with Aperture 3 being slow, were caused by problems with the initial setup:
- Upgrading the library will take a long time, if the library is large (reprocessing the raw, recreating thumbs and previews, indexing faces, ...)
- Aperture can be slow, if the library is corrupted and needs repairing.
- Aperture can appear to be slow, if corrupted media have been imported (incompatible videos or something)
- Aperture will be slow, if the Aperture library or the referenced master files are on a network drive and the access to the library is slow or the system drive does not have enough free disk space.
Added: Before upgrading have a look at the release notes: http://support.apple.com/kb/TS2518
Scroll down to the bottom of the page, where it says "Additional Information - A note on upgrading your Aperture library". That part is really important. Don't upgrade without repairing your Aperture libraries using your current Aperture version before the upgrade, and backup the libraries and the current Aperture version, so that you have an option to revert to your current setup in case something should go wrong.