Can keeping my CPU speed at “high performance/turbo” all the time decrease its lifespan?
And if so, what would be its expected route of failure if it does happen?
If the processor cooling system meets the required spec and keeps the CPU below its maximum thermal limit then there should be no issues. The main risk, as you may gather, is a catastrophic overheating, leading to a thermal failure but, in reality, most modern CPUs will start to lower their internal clock speed (to reduce heat generation) before this happens, but you may still find that an overheating processor leads to system instability and random crashes.
Overclocking your system (running the CPU at more than its rated frequency) can increase thermal instability due to additional heat generation and may increase the chances of an overheat condition if the cooling system is not uprated for the additional heat..
Overclocking will make your CPU hot so make sure:
-your temperatures are kept low
-the PC is kept clean to maintain these temperatures
As long as you keep the above in mind, overclocking it for a long time should not affect too adversely.
Actually, stressing your CPU more (and speed is a stress mostly because of heat) will make it age faster.
But this will not be noticeable because of a common property of most electronic chips that say their failure rate is low and does not grow for a very long time known as useful life, before they start to wear out. Stressing will only get the useful life shorter without increasing failure rate during useful life.
For CPUs, useful life typically lasts years, and they become obsolete long before the wear out.
You may want to look at the failure probability curve in this article I found: http://dependablesystem.blogspot.ca/2011/05/sw-and-hw-reliability.html
In short: stressing your CPU will make it wear out in 20 years instead of 25, but you don't care. Except for that, if it can be overclocked for minutes, it can stay for years.
From a purely technical perspective, the answer is yes. Transistors do age over time, and increasing the number of logic transitions (i.e. consistently high clock speed) or running a chip at a higher voltage does decrease the lifespan. This is due to the intrinsic material properties of using silicon-based transistors.
Unfortunately, this means that by keeping your CPU in high performance mode, you're running it at a higher clock speed and a higher voltage.
Fortunately, this lifespan reduction will be manageable if the thermal profile of the CPU is managed correctly. CPU lifespans are rated at full clock speeds and voltages for a certain thermal load. So long as you use an appropriate heatsink, and do not apply a voltage too high to the CPU, you should be fine.
Finally, it should be noted that CPU performance mode switching is usually seamless now. I would advise that you do not keep your CPU at full speed/voltage all of the time. This causes unnecessary power consumption, and thus excessive heat dissipation at idle.
To put it plain and simple, CPUs don't age. They keep performing at the same speed and same stability unless something damages the CPU. In most cases this heat. Liquid or environments that are too humid can cause shorts that will cause local overheating as well and damage its components. As with all matter in the universe, there's always some degree of aging, but with CPUs, this aging process is much slower than any other part in the computer, by far!
The only "aging" you will see in CPUs is that after a while, they can't keep up with evolution and newer, faster processors are coming out all the time.
In the old days, CPUs didn't have built-in thermal protection, so it was generally more common that after a while, the CPU overheated and had damage, thus needed replacement. In the last decade, I haven't seen a single case anymore where a CPU needed replacement, unless there was a factory fault with it and it didn't operate properly from the moment it got installed.
So, if you're getting a new computer these days, your CPU is the one part that will outlive all your other parts. This is provided you don't physically damage it, e.g. overheating or sending more electricity than it can handle by overclocking it beyond its limits.