I've got a couple of old phones that I've had for a few years, and while both of them are in great physical condition, both of them run very slowly. So slowly that it's difficult to really use them for anything; even the home screens tend to be sluggish. My suspicion is that it's some planned obsolescence thing where they run a version of Android that they can't handle, and I would like to put them on something where they'll run more smoothly. After few years of digging around on the XDA forums and trying different things, I've never really made any progress.
The phones are a Samsung Galaxy S4 Mini (SCH-I435) on Android 4.4.2 and a Samsung S7 Edge (SM-G935V) on Android 8. Both are Verizon phones, and my understanding is that they're locked down pretty tight; I've never found a solution that will let me root them or flash a custom ROM. The S4 shows an open padlock on the boot screen, so I think I was able to unlock the bootloader at one point, but I wasn't able to go past that. I was never able to get anywhere with the S7.
Is there any possible solution to getting these phones running more smoothly? I'm open to hardware and software solutions if any exist. Thank you.