Why is SSD writing slow?

Question

Windows

This is the speed on the M.2 SSD WD Green 500GB. Reading is fine, but writing fails.
SSD is in good condition, only used for a year.

SMART disk:

Answer 1

Smart is normal (at least what got into the screenshot), but with free space it's very bad.
SSD cannot be filled more than 75%, then the speed can drop catastrophically.
Try deleting 100 GB, wait for TRIM to work and check.