I've been noticing that cpu is pretty high in Windows 8 lately with Spark. I can usually get a full instance running and like 2 or 3 other synths without some stuttering. If I disable Spark I can keep on without stuttering.
I'm using a 3.3ghz intel core i5 quad, 16 gigs of 1800mhz ram, a ssd os drive, and a 10,000 rpm samples drive so I don't think it should be doing this. I've been hunting in windows hoping to find a setting that i didn't turn off that could be contributing ot the slow down.