Simon P is incorrect. His error is in only considering the processor/CPU. This is only one component. Effective speed is also altered by RAM, Disk space, operating system, configuration parameters (cache and buffering, etc.)
Though a Mac and a PC may have the same processor spec the Apple will be quicker. Why? The operating system seems to carry fewer overheads, plus the additional architecture and hardware tends to be better configured. For example, I believe the data bus for Apples tends to be better, so once the processor has done its bit the data gets to your disk or display more quickly on an Apple than on a PC (sorry, can't find a reference for this immediately).
Then there are other aspects of performance: crash rate, and ability to put a smile on your face.....
When you pay the apparently higher price up-front for an Apple it should be considered an investment - less time wrestling with Windows problems or suspect hardware later.
http://www.macobserver.com/article/2002/06/13.9.shtml
While nothing is perfect, Apple users tend to be happier about their computers.
http://www.pcmag.com/article2/0,1895,1851295,00.asp
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Some people say Macs are not so adaptable as PCs, or have weaker components. If this is the case, then why would processor and video intensive industries like Film Production use them. There is an activity monitor, and the "terminal" that allow direct interaction with low level processes - and it's much easier than on a PC. Tch!