[linux-audio-user] performance with AMD64

Hello, I am using Ubuntu Dapper on an amd64 athlon +3400 (laptop) with 2.6.15-26-amd64-generic SMP PREEMPT kernel. My audio interface is a RMC Multiface II / Cardbus. I've manually compile alsa 1.0.12rc2. I think the audio performance is way wrong : with jackd -R -dalsa -r48000 -p128 -n2 -D -Chw:1,0 nothing else, I get 1.2% - 1.9% CPU usage if I don't do anything. When I load zynaddsubfx it goes to 13%, again without touching the midi keybooard. By holding C major chord with the default sound zynaddsubfx starts, I get a CPU usage of 20%. with jackd -R -dalsa -r48000 -p256 -n2 -D -Chw:1,0 nothing else, I get 0.65% - 0.96% CPU usage if I don't do anything. When I load zynaddsubfx it goes to 7.2% - 8%, again without touching the midi keybooard. By holding C major chord with the default sound zynaddsubfx starts, I get a CPU usage of 11%. Could anyone please test something similar to my setup and post here ? I don't think this is normal. Thanks !