It's full an needs a TRIM command running to clean up all the half full clusters.
Writing one clump of data is requiring multiple fragmented clusters to be tidied before it can do a clean write.
Like having a million buckets all 95% full and you need to store 1 bucket of water. instead of filling one bucket in one go, it has to spread it over all the empty ones or shuffle the buckets around to clean one out for it.
Trim command is like Defragmentation for SSD, but basically your disk is too full. The chance of you having empty buckets is low.