I have a Samsung S3 that I bought about 3 years ago - it was on a good discount by then of course. In three years I have bought 2 new batteries neither of which have improved its speed or battery life. The S3 experience I have had bears no resemblance to the reviews of the S3 done in 2012. I have done several factory resets, tried many battery saver apps, tried multiple light launchers. While I have no way of proving it I would not be amazed if Samsung has written some code to subtly slow the phone down or to drain its battery.
I have a couple of S3s at home. The one I used worked perfectly and was more than usable. Another S3 which my partner used for a while was the opposite. I actually think they were different sub models. I keep meaning to check to see if the model number is the same, might be an overseas version etc.
I previously had an S3, and found it to be terrible, that I ended up mainly using it solely as a phone. It got so slow over time, and the plastic build quality meant that it cracked in places, including around the jacks. Apparently some had a memory issue with a software patch was supposed to fix, so not sure if that was related to the problems I had. Not sure if it was battery related either, but never needed to replace it, as it had good battery life. But if I factory reset it, it worked fine for a few weeks, and then slowed down. Android has improved a lot since then though, but wouldn't buy another samsung. The iphone I have foud to be pretty quick and responsive, at least up until ios 11, which appears to have has slowed down the 5s a lot from my experience.