Just finished loading Omnisphere and all the updates. I'm on a Dell Core 2 duo with 3 gigs ram. Using Sonar 5 PE. (just never bothered to update). It loads in about 8 seconds and patches load in about 2 seconds. There is so much here to explore.
Update: I loaded one of the larger patches and used Altiverb to see what would happen. CPU ran about 16% when playing quick notes in succession. To me it seems about the same as GVI load wise. Haven't tryed Live Play or Stacking yet.