What operating system are you using?
Can you verify that TRIM is enabled? (it should be and would be strange if it were not)
What is your current disk utilization looking like?
Have you already ran Disk Utility First Aid?
Have you pulled SMART data to check the drive's health for any pre-failure warnings?
I find the Blackmagic app does not always give the most consistent results. Try the AJA System Test and see what sort of results you get: https://itunes.apple.com/us/app/aja-system-test-lite/id1092006274?mt=12#
I have noticed if I let it fill up to less than about 80GB free space things slow down a bit. Maybe try freeing up some space ?
Hmm, mine is only a 256GB drive. After making some room, I now have 53GB of free space, which means over 20% of it is free
I restarted my machine, re-ran the test and my write speed now settles at around 249MB/s for a 2GB file test, and around 180MB/s for anything bigger.
I can't really make more room, but it's odd that it should require so much free space. It also seems odd that write speeds start so high, but then start to come down pretty quick.
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Try holding command-s at boot to start in single user mode. Then enter "fsck -fy" (without the quotes) at the command prompt. That should TRIM the drive and maybe speed things up. Just type reboot after to restart.