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One little-noted feature of the G5s is something called "processor and bus slewing." When it's enabled -- as it is when you take a new Power Mac out of the box -- the system doesn't run at its rated speed all the time.
To keep power consumption and the resulting heat and fan noise to a minimum, the machines operate at reduced voltage and speed when they're idle or performing light work. Only when they face heavy lifting do the processor and bus kick into high gear.
According to the technical documentation about the new series that Apple supplied to developers, shifting processor and bus speeds "does not impact system or application performance." But we, like other benchmark testers, discovered that this isn't quite accurate. When we went to System Preferences, opened the Energy Saver preference pane, clicked on the Options tab, and switched the Processor Performance setting from Automatic to Highest, the scores on many of our benchmark tests improved by 3 to 7 percent, and we noticed no significant increase in fan noise. (We used the Highest setting for all the tests reported in our benchmark chart.)
In this mode, the G5 consumes more electricity, which isn't exactly friendly to the environment or to your wallet. But when you need maximum performance, you might decide the costs are worth the candle.