My iPhone XS overheats, feels slow and has below average bench scores.

Hi everyone,

For some context, I bought a refurbished iPhone XS about 6 months ago. Everything seemed to be fine, but since about a month ago several issues have appeared. First, the iPhone overheats quite a lot, especially near where the CPU is.
Second, during random moments everything seems to slow down a fair bit, animations become laggy and the phone even froze completely while watching a video.
So I ran a Geekbench test to see if anything was wrong (several, actually), and although the single-core score seems fine (1110 approximately, stays even throughout testing), multi core scores are all over the place, ranging from 1900 to 2800, when the average is 2500. I know benchmarks aren’t everything, but I find it a bit suspicious that scores vary that much.
Help would be greatly appreciated.
thanks!