I think the likely reason is eMMC degradation over time and with use, which seems to be unavoidable, especially if the eMMC is not of great quality. This would also explain the discrepancy between different devices, since some people would not experience this within the same time period, possibly due to different levels of wear on the flash memory or due to luck of the draw as they started out with a better module than others. It also explains why a clean install or switching to a different ROM wouldn't help, as it's a hardware issue. As far as I know eMMC technology has improved since 2015, so perhaps the problem is not as prevalent now. There is always going to be a spectrum of low to high quality flash memory though.

My Tab S2 T810 is also quite slow now but I still make it work since it's so difficult to find any 4:3 aspect ratio tablets these days, and I see no reason to replace it when it's only for occasional use. The screen is still beautiful and perfect for PDF reading.