The SW1000XG does not store its audio parts onboard, they are stored on your hard drive, so the problem is with playing them off your hard drive, This will take cpu time and also depends on several things, mainly the speed of your hard drive, if it's optimised for audio, the bandwidth of the bus from the hard drive etc. The card itself handles the midi and realtime fx which takes no cpu at all, and if you are running the audio masters and groups etc in Cubase then it will start to slow things down a bit, but this is not the fault of the SW1000XG. A standard H/D will spin at about 7800rpm an audio optimised H/D will spin at about 10000rpm so you can imagine the difference in transfer rates. but the more audio tracks there are in use then the more cpu time is required for each one.
hope this helps.