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My relatively new i7-4790K @ 4.00GHz has been exhibiting terrible performance since I bought it a few months ago.

The machine feels extremely sluggish. CPU utilization at rest, as reported by SysInternals Process Explorer is usually around 7%, and if I do as little as to run a browser and open up a page with a video or a little animation in it, it jumps to 30%; everything I do has considerable delays, for example IntelliJ IDEA has constant lag while typing; and most importantly, PassMark PerformanceTest is giving me CPU and Memory scores that are noticeably below world average.

An Intel i7-4790K at 4GHz is not supposed to be anywhere near world average. It is supposed to be far above that.

While troubleshooting things I ran Intel's "Extreme Tuning Utility" and saw that the machine is usually running at 800 MHz instead of 4 GHz, but it briefly jumps to 2.5 GHz or even higher when it has some serious work to do, but only on extremely rare occasions and for very brief periods of time. Also Intel's utility shows that no heat or power throttling whatsoever is taking place at any time, no matter how much I am taxing the CPU. So, cooling is not the culprit.

I tried various other things, like switching off Aero on Windows 7, to no avail.

What could be the cause of this terrible performance?

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    It's not clear in the question, did you upgrade to this CPU or was it a whole new computer? – James P Oct 11 '16 at 14:06
  • @JamesP excellent question. I upgraded CPU and motherboard together. I do not remember whether I reinstalled the OS. (I doubt I could have avoided that, though, could I?) – Mike Nakis Oct 11 '16 at 16:41
  • @JamesP I wonder how I could find out. There are directories in my Windows folder that date from 2009, then there are directories dated from 2015-02 to 2015-09, (the upgrade happened around 2015-09,) and then the next set of dates is current. – Mike Nakis Oct 11 '16 at 16:46
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I have found one acceptable (but not entirely satisfying) solution to the problem:

I went to Control Panel -> Power Options -> Change plan settings -> Change advanced power settings -> Processor power management -> Minimum processor state and I changed it from 5% to 100%.

All the sluggishness disappeared, at the cost of 10 more watts of power consumption when idle. PassMark PerformanceTest finds my machine to be at 94% compared to the rest of the world, and the machine certainly feels like that: CPU utilization when idle is down to 1%, and there is no annoying lag anymore, at all. I am so happy.

The thing is, I cannot understand how this can be, so I am afraid that there may be something more sinister at play. A "Minimum processor state" of only 5% is quite common in power profiles of modern computers, and of course it is not causing the kind of sluggishness that I have been experiencing.

So, it appears that on my system, the module responsible for controlling the throttling of the CPU (whatever this may be, hardware, BIOS, windows, I don't know,) is doing a terribly bad job: it is supposed to allow the CPU to run at maximum speed when needed, but it never does, not even when running the PassMark benchmark.

So, I am wondering: what is the root cause of this behavior?

Another strange thing is that the "Windows Experience Index" is reporting roughly the same numbers now as before: about 7.8 for the processor, 7.9 for the memory, and 6.6 for the graphics. (The mediocre graphics performance is mainly because I am using the CPU's integrated graphics for the time being.) This could be because Microsoft's benchmark is clever enough to enable full CPU while benchmarking and restore it once it is done, but I do not know that it does that for a fact, so it could also be an indication that something else, more strange is going on.

Feel free to add an answer if you have any more insights on this.

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