Ah yup eMMC https://www.gsmarena.com/samsung_galaxy_tab_active2-8897.php

Yeah, then copying the files most likely moved them to different part of storage and it improved performance again.

Sadly that's a common plague for cheaper Android hardware - after enough writes the flash performance drops off a cliff making those devices essentially unusable :/

(More expensive hardware - including Apples - tends to have UFS type storage which lasts longer.)