Comment by alex23478

This happened to one of my previous Xiamoi Phones too. Apparently the Flash Storage degrades over time, which makes everything that uses I/O super slow. For instance, starting Firefox took almost a minute, but it was usable as soon as all needed data was available in RAM. This is a known issue in the respective community, and sadly kills devices that would still be usable with the latest Android thanks to Custom Roms.