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). Screenshot of Androbench

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!