Revisited this oldie today. 5 instances, ~45 polyphony, all 2x oversampled = 30% CPU usage, which includes all insert effects. They don't make synths like this anymore. One instance of Diva alone reaches 50% at high polyphony.
It's not really fair to compare this to Diva. They use different ways to make sound. Z3ta is a wave table synth and Diva is a virtual analog synth that what I think recreates the circuits of an old analog synthesizer. That takes a lot more processing power. Also you can change the quality in Diva. But I agree with you. Z3ta is an awesome synth.