Samsung Galaxy S6 4.6MB/s Seqential Write (Why so slow?)

I bought a Samsung Galaxy S6 in Aug 2016. Everything was fine, until last week.
It got very slow. For example, when I draw my unlock pattern after a reboot, it takes such a long time to load the homescreen that the screen turns off. After a few tries, I get in but every tap takes very long until the device reacts. It takes ~25 seconds just to load the keyboard.

I downloaded System Monitor but the CPU load was not on 100%, so there is no app/virus which consumes too much CPU time.
Geekbench 4 says that the CPU / GPU performance is even above average for the Galaxy S6.

But then I decided to test the storage speed. I ran Androbench and the results are very bad (see screenshot).

Normally, the performance should be like in this test.

So my questions now are: how can I find out why is the storage so slow?
Is it just flash wear? Is is a broken driver? If it's a hardware failure, what part do I have to replace?

If something is unclear, just comment and I will edit the question.
Thank you in advance!

UPDATE 2019/06/20 08:03: Tried reflashing the stock firmware and factory reset, but it didn't helped. The benchmark is even worse now. Ordered a new motherboard now.

UPDATE 2019/08/19 09:57: New motherboard finally arrived. After changing it, everything was fine. Problem solved.