One contributor is that most models get a mid life major Android version update after 2 ish years and those don't work as well as the launch versions. It's unli

One contributor is that most models get a mid life major Android version update after 2 ish years and those don't work as well as the launch versions. It's unlikely to be hardware problems you suggest. Flash ftl slowdown due to filling up or fragmentation from use might contribute though. There are some jank debugging tools on Android (eg system tracing found under dev settings), would be interesting to see what they report on a fresh vs slowness afflicted device.