Possible issues
Dittos. No problems with Aperture after upgrading to Lion. You might check a couple of things, as disabling these helps. If you had Mobile Me and switched to iCloud, Aperture still tries to connect to Mobile Me. You have to close your Mobile Me account in Aperture (caution: it will delete your online albums)
Second, if you use Aperture to post to Facebook, Aperture will check all your Facebook photos you've EVER posted. That's background, but makes Aperture sluggish until it's finished.
Third, I work professionally, so have about 30,000 photos in Aperture. Once a year, I like to archive and start a new Aperture library.
Fourth, I also turn off "places" as Aperture uses your GPS info to attach your photos to maps on a GPS enabled camera, like the one on the iPhone.
Fifth, I turn off facial recognition. All these features work in the background and can make Aperture sluggish, although I have no idea if any of them are related to your issue.