A Small Group of CPUs from the Past
CPU technology has grown by leaps and bounds over the years. In just shy of three decades, we have gone from simple 16-bit single-core CPUs running at 4.77MHz, to incredibly complex 4-core CPUs running at over 3GHz per core.
CPU Architecture
CPUs of current technology are incredibly complex pieces of engineering. For a long time, it was assumed that more MHz meant better performance, a faster CPU. In 2001 this was called into question and shown to be a myth. Because of architecture evolution, MHz is only a part of the equation. A CPU running at 1GHz using today's cutting-edge architectures will run laps around a 1GHz CPU from just three generations ago.
Bus Speed
Bus speed is another factor in the overall performance of a CPU. The bus dictates how fast the CPU communicates with the rest of the computer. This is done with multipliers and dividers. Example: Athlon XP CPU at 1.33GHz. That is using a 133MHz bus, with a 10x multiplier. The CPU communicates with the RAM at 133MHz, and then through dividers, speaks to the AGP and PCI buses at slower speeds. Newer CPUs use clock doubling and multiple channels to obtain even higher CPU speeds.
Core Speed
This the raw speed at which the CPU operates. Today's CPU cores operate in the GHz range, performing billions of operations per second. The architecture that the CPU is built upon has a lot to do with how efficiently the CPU performs these operations.
Bus Architercture
The bus the CPU and all of the computer's peripherals run on also has an impact on overall CPU performance. An inefficient bus will cause delays in transactions between peripherals and the CPU. A slow bus will simply hinder things by being slow, though a slower bus has also been used to purposely create different CPU speeds to fill different needs.
Cache
All CPUs have what is called on-die cache. This is small amounts of RAM that run at or near the speed of the CPU to speed up the process of computing, and to keep the CPU "fed" so that it is not wasting any time doing nothing. Having an efficient cache makes a huge difference when it comes to performance.
Conclusion
One could argue that if two cores are better than one, then three must be better than two. But based on the information just outlined, it should be easy to see that this could quite conceivably be false. All it would take is a poorly designed cache, bus or architecture, and a triple-core CPU could find itself trounced by a dual-core.
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