Old iphones become faster if you reboot them, like most devices. This is true whether or not you change the region. There's no qualitative difference when you test repeatedly.
https://knowtechie.com/want-to-speed-up-your-old-iphone-try-...
https://knowtechie.com/want-to-speed-up-your-old-iphone-try-...
I was under the impression that technology doesn't "just age". The software running and apps being updated for new platforms might make it feel "slower" but in general using the phone itself shouldn't ever feel like it's aging.