As stated in a few other answers, CPU manufacturers want to keep clock speeds down to control power consumption and heat dissipation. In order to do more work at the same clock speed, several strategies are used.
Large on-chip memory caches can keep more data "close to" the CPU, available to be processed with minimal delay, as opposed to main memory, which is much slower to deliver data to the CPU.
Different CPU instructions take differing numbers of clock cycles to complete. In many cases, you can use a simple circuit to implement an operation over several clock cycles, or a more complex circuit to do so in fewer.
The most dramatic example of this in the Intel evolution is in the Pentium 4, which was a big outlier in clock speed, but didn't perform proportionally well. The bit-shifting instructions, which in previous chips could shift 32 bits in a single cycle, used a much simpler circuit in the Pentium 4, which required a single cycle for each bit shift. The expectation was that the Pentium 4 architecture would be scalable to much higher clock speeds because of its simplicity, but that didn't work out, and the fast, complex shift circuit returned in the Core and later architectures.
57The desktop chip is a dual-core; the server is a 16-core CPU. Using a single-threaded benchmark is NOT appropriate at all. – MSalters – 2013-07-08T09:58:01.277
@learner - Because the speed of the processors cannot increase with increasing the amount of voltage they required which reduces heat. By concentrating on power consumption they will be able to in theory ncreased the processor speeds in the future. – Ramhound – 2013-07-08T11:04:31.147
1Please cite actual cases of 'manufacturers' 'even reducing' clock speeds (without comparing apples and oranges) or limit your question title to 'not increasing'. – Jan Doggen – 2013-07-08T15:06:00.480
8AMD vs Intel clock speeds haven't been a fair comparison since the K6/Pentium days. AMD marketed Athlon processors as 2500+ or 3000+ when their core clocks might be 1.8 or 2.1 respectively, but they typically benchmarked quite respectively with Intel chips that did clock a true 2.5 or 3GHz. There are simply way too many differences between architectures now to make a comparison simply on clock rates. – KeithS – 2013-07-08T15:35:57.557
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Related: Has CPU speed already broken Moore's law?
– Rich Homolka – 2013-07-08T17:40:12.333Execute 16 of them at the same time and you should start to see a difference... ;-) – Marcello Romani – 2013-07-08T22:59:01.487