Why do Macs have slower processors?

"The old Pentium 4 processors were faster" Don't let the numbers fool you. The Mac processors & most processors in recent computers are Duo Core. Essentially they are 2 X 1.83GHz processors working together.

The dual core processors made by Intel all have lower clock speeds. Basically, it's tricky cramming two processors into the same-size package and lower processor clocks are the result of trying to mitigate the effect of the higher-density chips.

Mind you you have 2 x 1.83 GHz processors = 3.66 GHz (more or less) equivalent. An iMac with 2 x 2.33 GHz CPUs is 4.66 GHz...

Incidentally, this is not particular to Macs, it's also true of PCs. It should also be noted that at the same clock speed the Intel core-series processors outperform the Pentium4 processors (which were pretty awful).

I believe that I have read that Apple chose to use Intel processors designed for laptops, in both the iMac and its laptops because they run cooler, and draw less power. This may also account for the lower clock speeds.

Is the Mac OS written to take full advantage of two processor cores? Seems that's what I'm hearing? This is an important question for me, trying to decide which Mac to buy. I'll be wating until Leopard anyway, but I've started doing my 'due diligence' before spending.

UNIX-based
Beneath the easy-to-use interface and rich graphics of Mac OS X lies Darwin, an open source, UNIX-based foundation built on such technologies as mach and FreeBSD. With Tiger come significant enhancements to this UNIX foundation, including improvements to dual-processor support for increased performance when reading and writing files to disk and when using Mac OS X with NFS file servers.

Have you noticed that many processors in PCs have
faster clock speeds (gigahertz) than many in Macs? I
have an iMac with only a 1.83 GHz processor. It
seems like that sounds slow. The old Pentium 4
processors were faster. Is it a different scale, I
think I heard that somewhere?

Comparing a Pentium4 with the new Core processers solely on clockspeed is meaningless.

The Pentium4s had high clockspeeds, yes, but they were also power hogs, ran hot, and did less per cycle -- that's why they could be ramped up so high. In comparison, the Core processors use relatively less power, run cooler, and are more efficient from a performance per watt standpoint.