Very slow write performance 30min+ for 2GB - APFS?
Okay after running 

diskutil APFS defragment /dev/&lt;yourdisklocation&gt; status


I saw that defragment was disabled.
Turning it on with 

diskutil APFS defragment /dev/&lt;yourdisklocation&gt; enable


an waiting about an hour, the write speed startetd to return to "normal".
I could Copy the 2 GB in about 5 Minutes instead of 40´

Anyway - the Cruxial 500 MX 1TB SSD ist already in my Postbox.