I thought the "upgraded" OS slowed the phones down to restrict battery use of their non-replacement batteries that were dying...i.e. planned obsolescence.
You thought wrong. Most upgrades speed up the operation of the iPhone. What slows them down is the dying battery which can be easily replaced by Apple or many third-party companies, or even the user themselves with kits including tools bought through Amazon. For some models, it can take less than five minutes. Instructions are available on YouTube. Apple does slow down the older phones with chemically depleted batteries to prevent the batteries themselves from automatically shutting down through internal battery safety circuitry which is present to prevent overheating or fires due to the fact that older lithium ion batteries will continue to provide the same voltage by pushing higher and higher amperage and HEAT as the chemicals get depleted as the device tries to draw the same requirements.
Apple opted to slow down the clock speed as the battery gets to the point it cannot provide what is demanded to spread that demand out over a longer period of time so as to maintain the operation of the device for emergency communications at the expense of less necessary apps such as games. Replace the depleted battery with a new one and the phone returns to normal speeds.
Replacement battery kits can be purchased for as little as $12 on Amazon. Some third-party replacement locations such as Batteries+ will do it for you while you wait for as little as $50 - $60.