Slowness, reboots, heating and (as a result) reduced autonomy may indicate a problem with SD card and / or SIM. The phone is “doing more” for some reason.
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Begin by cleaning the contacts of the cards and in their sockets.
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If you have a SD card make sure it’s formatted as removable / portable and not as internal memory expansion (more important info here).
If “internal”, take steps to make it “portable”.
If “portable”, try removing it and observe how the phone behaves. -
If the SIM is more than three years old, you might look into replacing it. Failing SIMs cause lots of problems.