This is a phenomenon I've been curious about and my intuition has always told me this should be expected. Today I did a quick test to see if my tuition had any merit. This has been brought about because some people have been saying the newer laptops such as the MacBookAir can out perform some older MacPros. My test was done on my MBP8,3 (17-inch late 2011). It's rated at 2.5 GHz and has an Intel Core i7 processor with 4 cores. The question being explored here is can this MBP8,3 maintain at least its 2.5 GHz when placed under a heavy workload. The processor is sensitive to heat and its hardware architecture is designed to keep it within a certain power/heat envelop. So I started the Intel Power Gadget with just a light load and slowly increased the workload to cause all 8 core threads to be in operation. You will note that as this heavy workload comes on board the processor hardware will step in to reduce the core clock frequency below the rated 2.5 GHz. This shows that a laptop such as the MBP8,3 will not be able to achieve a constant 2.5 GHz of compute power when placed under a heavy workload of any reasonable duration. Now for a real workstation such as the MacPro (old and new ones) this should not be such an issue and will run a similar test later on using a new MacPro6,1 12-core with a stated processor clock rate of 2.7 GHz. As soon as the 8 cores in my MBP8,3 start operating flat out the two fans very quickly ramp up to their max RPM of 5500. They stay at this RPM so long as the cores are running flat out and only reduce their RPM as the cores start idling down. Thus, max cooling is employed for the processors when they are under constant heavy use. My test shows the MBP8,3's cooling capacity allows my processor to maintain around 2.0 GHz when under extreme workloads. Thus from a performance standpoint when deciding just how much work the MBP8,3 can perform when dealing with a heavy workload I should be using 2.0 GHz as a point of reference for getting work done, and not the rated 2.5 GHz. It will be of interest to me to see how this type of test pans out on my MP6,1 12core later today. ...later The test that represents the screen shots posted above was for a heavy workload running for maybe 10 minutes. I wanted to show in the Intel Power Gadget's display what the GHz were for a light load and then bringing onboard a very heavy workload. I allowed the heavy workload to run for 30 minutes after and the result was as shown in my OP. The GHz simply was being maintained at below the rated 2.5 GHz for the processor. I have now been able to run a similar test on my MP6,1 12-core with a 2.7 GHz processor and 64GB RAM. Here are the results with a light workload transitioning to a very heavy workload. This transition is clearly seen in the top graph showing the MP6,1's power consumption. Note that the TDP for the MP6,1 is 130watts. Under the light load the power consumption was level at around 40 watts, fan spinning at 790 RPM, and as the heavy workload was brought on this power consumption stepped up quickly to around 100 watts and the fan speed steadily rose up to around 1000 RPM. Note that as the power transitioned from 40 watts to 100 watts the Processor cores stayed reasonably level at around 3.0 GHz and just above the processor's rated 2.7 GHz. I left the heavy workload running for 30 minutes. The Processor kept chugging along at an average of 3 GHz with an occasional dip to 2.85 GHz. The fan RPM stayed constant at around 1000 RPM. All 12 cores were running flat out at 100%. Processor temps held at around 160ºF. I also had heavy i/o active at around 320 MBytes/sec to a RAID-0 LaCie 2big Thunderbolt-1 device. This test and the results are a testament to Apple's excellent design for this MP6,1 and tells me it's a workstation system that can run a heavy workload all day long, all week long and maybe all year long without taking a breath. No Apple laptop can do this type of computing workload without reducing its CPU operations to stay within its TDP and will in fact run at a performance level less that its stated Processor GHz. I am pleased to see the MP6,1 12core processor rated at 2.7 GHz can in fact run all day long (I assume this all day long even though my test ran for 30 minutes) at around 3.0 GHz and not some lessor value below 2.7 GHz. The MPs are real work horses for sure and can run all day long doing heavy work, and hopefully you will agree that the laptops simply cannot match this type of sustained performance. ...Oh, and BTW the heavy workload I ran on the MBP8,3 and the MP6,1 were very very similar... as I used the same applications in each testing scenario. Here's a performance set of data as the heavy workload was brought on... the Processor temp and fan speed were starting to ramp up... ...and here is what happened after the test completed with the heavy workload gone and returning to a light load. Here's a test (no i/o was involved -- purely CPU) showing clearly how turbo boost kicks in when a single core is heavily used on my MBP8,3 running with a stated 2.5GHz Processor that has a TDP of 45watts. I started the single core activity and then terminated it after a short while so that the graph showed how the boost kicks up the GHz. Note that the single core does not reach it highest turbo boost value which should be 3.6 GHz. It instead goes up to 3.3 GHz only.. I also ran this same single core test for around 30 minutes and the MBP8,3's two fans spun at 5500 RPM constantly and were able to keep the processor temperature at around 92ºC, thus allowing the turbo boost for the single core to be maintained. The power wattage was averaged 30 watts with an occasional spike to 45 watts. I was never able to get the single core turbo boost to kick the clock rate up to 3.6 GHz, even with every thing else terminated. I simply suspect some processes were demanding some CPU attention. I ran another test for well over an hour with all 8 threads being used 100% CPU. The core rates were reduced to an average of 2.2 GHz, the power held at around 30 watts, two fans constantly running at 5500 RPM and Processor temperature steady at 90ºC. This tells me that if I want to run heavy CPU intensive workload on my MBP8,3 I should reckon on not more than 2.2 GHz for the cores. ======== Also, here's some additional information on TurboBoost. http://www.intel.com/content/www/us/en/architecture-and-technology/turbo-boost/turbo-boost-technology.html which does have the statement: Intel® Turbo Boost Technology 2.01 automatically allows processor cores to run faster than the rated operating frequency if theyre operating below power, current, and temperature specification limits. Intel Turbo Boost Technology 2.0 is activated when the Operating System (OS) requests a frequency higher than the rated frequency of the processor. Whether the processor enters into and the amount of time the processor spends in the Intel Turbo Boost Technology 2.0 state depends on the workload and operating environment. Maximum turbo frequency indicates the highest possible frequency achievable when conditions allow the processor to enter turbo mode. Intel Turbo Boost Technology frequency varies depending on workload, hardware, software and overall system configuration. Due to varying power characteristics, some parts with Intel Turbo Boost Technology 2.0 may not achieve maximum turbo frequencies when running heavy workloads and using multiple cores concurrently.