I have an iMac 5k Retina with an i7 6700K CPU. This CPU is rated 4.0GHz base, 4.2GHz Turbo. The data sheet is here. This amazing hardware, with a stellar display, and with VMware Fusion, a real productivity booster.
Running 2 x Ubuntu VM's with compile jobs, which saturate all 4 cores (8 Hyperthread CPU's) at 100%, get CPU throttling 3.7GHz. !! Intel and Apple both claim the CPU shouldn't fall below 4.0GHz. What gives?
Core temps should be below 80C, mine is baking at 95C, with fans on maximum. Power consumption is about 60W, well below the TDP of 91W.
My guess has to do with a sneaky feature in 6th gen Intel CPU's, see Section 5.1.4 in the detailed specs here. This allows a vendor, say, Apple, to set the BIOS to a lower/throttled power limit, say, 60W instead of 90W. If the CPU is pushed harder than the reduced limit, then a different kind of CPU throttling kicks in. Sneaky, eh? Section 5.1.7 also shows a variable thermal throttling too, but that would kick in at ~80C? So I think we are seeing 5.1.4 - type limiting. From the test below, the throttling appears to be either around 95C or 60W, it's hard to tell which throttle mode is in effect.
The compile jobs took 51 and 52 minutes in my VM's on the iMac, with 4 (virtual) core CPU's each and 8GB RAM each. The exact same compile took only 44 minutes, on an Amazon AWS c4.xlarge EC2 instance. This is also a 4 (virtual) core CPU system, with 7.5GB RAM. EC2 has better bandwidth than my home, but not by tons. I'm getting 125 Mbits/s down, and these EC2 instances get about 160 Mbits/s in the real-world compile benchmark. Apple throttles their hardware. Amazon doesn't seem to throttle EC2 c4 instances.
iMac cooling is insufficient, so they probably throttled the CPU in BIOS. Also, Intel is not truthful about base frequency. *sigh* At least Amazon has truth in advertising. Amazon EC2 for the win!
The video below shows the baking happening. Also, you can see the graphics load start to wake up as I am recording the video. This slows the CPU from around 3.8GHz to 3.7GHz, or average 3.9GHz to 3.8GHz.
The video has no sound, so you can't hear the fans whirring like crazy!