Ridiculously Slow Macbook Pro 15inch Early 2013
I have yet to see a horrible, measurable slow down, but that is exactly what is expected as SSD wear out. 


What happens at the user/OS level when an SSD finally wears out?


If you have an easy SSD handy to swap, I would get a black magic disk test benchmark before and after the swap.  

If you don’t, perhaps get a benchmark of disk IO and then erase the drive after backing up and repeat the test in a fresh os install that is mostly empty of user data. 

Once you can post some specific read and write speeds, that would let other people chime in if you're in the same ballpark for speeds.