Decreased transcoding speed

Does anyone have any ideas about why my transcoding speed would drop? It was taking me about 8 minutes to transcode a DVD, now it takes about 17 or 18 minutes.
System specs:
AMD 64 X2 4400 +
Abit AN8 32X
2 - EVGA 7600 GT Superclocked SLI
2 gigs corsair xms memory
WD Raptor 74 G
Plextor PX716A

Depends on the movie size, and how much % you get in the quality bar?