No, it only slows down the older iPhones that have chemically depleted batteries IF an application is making an extraordinary demand on the battery by putting a large demand on the processor or GPUs. These are the power-hungry components of the mobile devices. . . and a chemically depleted battery cannot supply the demands that it once could when new. It rapidly reaches its ability to do a phase-change. It WILL keep providing the amperage draw, but to do that the VOLTAGE drops, causing heat in the battery to rise. The internal circuitry of the battery will note these imbalances and will eventually shut down the battery to prevent swelling and/or fire, cutting off all power to the device.
Apple inserted an algorithm to monitor this power draw of processor/GPU intensive apps and to prevent these over-drafts, spread their processing out over a longer period of time to prevent these shutdowns. This has the effect of making these particular apps run slower so they will not draw as much power from that depleted batter. Replace the battery with a new one and the algorithm will not see an overdraft condition reported from the battery so it will not need to spread that app's processing out over time. This is exactly what has been reported every time a chemically depleted battery has been replaced with a new battery.
Yes, it is possible to make this a user selectable option IN ADVANCE of the shutdown event, but not a choice at the time of a shutdown, because there is no warning when the battery decides to shutdown. That is something where no warning is given to the device. That happens entirely internally in the battery. Once decided, it is the default position of the device.
Once the device is shutdown by the battery, it will NOT restart until the device is placed on a charger for a few seconds to reset the internal battery circuitry. The batteries that shutdown in this manner usually have an adequate charge in them to run normal apps. . . it is just the over drafting of power by these processor/GPU intensive applications that cause the shutdowns.
One of the reasons for that is that older devices were built with dual core processors and a single GPU. Later devices have four to six core processors and multiple GPUs with multiple cores, all of which are between four to eight times faster than the older devices.
Developers create or update existing apps to take advantage of these more powerful processors and GPUs with multiple cores, and even the upgraded iOS is so upgraded. . . and expects to do much more in processing and moving graphics around more quickly. The older devices do what they can to do the same, but try to do those jobs with a double core and a single GPU what the newer devices are easily handling with multiple cores and multiple GPUs. . . and to do that the older less efficient devices demands more power from a depleted, older battery that just cannot provide it anymore. The battery tries to do it, but can't and shuts down.
Ironically, the very test that demonstrates the speed of these devices, GeekBench Testing, stresses the processor and GPUs in exactly the same way as the worst of these processor/GPUs intensive applications do. . . in fact, it is designed intentionally to do just that. Ergo, Geekbench will trigger Apple's algorithm to throttle itself to spread the TEST out over time, showing the slowest processing to keep the device from shutting down due to battery overdraft. The very act of testing CAUSES the slowdown to occur. . . by design. This is Swordmaker's Heisenberg Uncertainty Principle for iPhones. The test is part of the problem, because it creates the symptom!
So, if the user switches off the throttling, he is likely to wind up with a bricked device until he can put it on a charger to resurrect it. If he allows it to be throttled, it will work, but slower on some heavy duty apps (but more and more normal apps are becoming "heavy duty" as more bells and whistles are added by developers. Damn rock and hard places. The real solution is to replace the battery.
Wow! Thank you for that explanation. It makes sense. And it doesn’t make Apple seem like an evil and greedy engineering obsolescence intensive corporation.