3 Tone Down the Graphics
This may not be an ideal option for many people, since it pretty much degrades the smooth animations and other aesthetics of iOS. But it’s worth a shot if other methods aren’t having an impact on your iPhone’s performance.
Just go to Settings > General > Accessibility > Reduce Motion, and make sure the option is enabled. (This doesn’t free up any memory, it just speeds up app loading.)
Similarly, go back to Accessibility, tap on Increase Contrast and enable Reduce Transparency.