My old iPhone was laggy until I toggled this setting

I switched to an iPhone 16 Pro a few months ago, but I still keep my old iPhone 12 around. It’s far from retired. These days, I use it to test new apps, take it with me on runs, and sometimes even as a webcam for my PC. It still works fine, but after spending time with the new iPhone, it definitely didn’t feel as smooth or responsive anymore.