I have an i5-2500K CPU in my PC. This particular CPU is rather famous among PC enthusiasts in that it's amazingly overclockable. Its official base clock rate is 3.3 GHz, with an official maximum boost frequency of 3.7 GHz (which is a technique supported by more modern Intel processors where, if enabled, the CPU will automatically and dynamically "overclock" itself depending on the current load.)
However, with proper cooling, people report the CPU being completely safe to be overclocked even up to 4.6 GHz without problems, which is rather astonishing.
I have an efficient CPU heatsink by CoolerMaster, which keeps the CPU incredibly cool even under full load. With this heatsink, without overclocking, the CPU stays, when idle, at about 35°C and even under. At full load (all four cores at maximum load) the temperature is under 55°C, which is quite remarkable.
I have overclocked the CPU to 4.2 GHz, and under full load the temperature is about 63°C, which is still well within completely safe limits.
A friend of mine has a Xeon E3-1281 v3, and he uses the exact same heatsink as I do. His CPU is not overclocked, yet it reaches temperatures of even 80°C when under full load. We have been pretty much unable to determine what the problem may be. However, I have a theory about this.
The main reason why CPU's have a limit on clock speed is temperature. (There are, of course, other factors at play, but temperature is, by far, the major one.) When the CPU is overclocked, its temperature under load inevitably increases, and it can only take so much before it breaks (although with basically all modern processors, before it throttles down to avoid breaking.)
One of the reasons cited on why the i5-2500K is so famously overclockable is that it has a really well-crafted heat transmission between the CPU die itself, and the heat spreader lid that's on the CPU chip (which is what then makes contact with the heatsink; or, in this case, the heatpipes of the heatsink.) This allows the heat from the CPU die to be efficiently transmitted to the heatsink, keeping the CPU relatively cool even under heavy loads and massive overclocking. With an efficient heavy-duty heatsink this means that astonishing overclocking (such as from 3.3 GHz to 4.6 GHz) becomes possible.
Many people have noticed, however, that more recently Intel processors not only run hotter, but also cannot be so massively overclocked, no matter how efficient the heatsink may be. Even if you have a really, really massive heatsink made of the highest quality materials, it doesn't help much. Moreover, the CPU seem to get worse and worse in this regard in just a couple of years, as if its heat dissipation capabilities degraded over time.
And the reason for this becomes clearer when the CPU heat spreader lid is removed: For some reason Intel has started using really cheap thermal paste under the lid (ie. between the CPU die and the lid). There are videos on YouTube about people doing this to a modern Intel CPU that is running really hot, and discovered that the thermal paste inside there has pretty much solidified and lost most of its heat transmission properties. This is something that happens with really cheap, low-quality thermal paste. When the junk paste is removed and replaced with fresh high quality thermal paste, the temperature goes way down.
Why would Intel suddenly start doing this? Even the highest quality thermal pastes out there are relatively cheap. We are talking about something like less than a US dollar per CPU. These are CPUs that cost from 200 to over 500 US dollars. One meager dollar more isn't going to change that by much.
My theory (and I'm not the only one) is that this is planned obsolescence by Intel. They are deliberately putting low-quality thermal paste in their CPUs so that they will become unusable in a few years, forcing the majority of people to then buy a new one.
The i5-2500K was, possibly, one of the last CPUs that was explicitly designed by Intel to be highly overclockable and durable, thanks to a fantastic high-quality heat transmission between the CPU die and the heat spreader lid. After that it seems that they changed policies and went for a really, really underhanded anti-consumer tactic of planned obsolescence.