Advertised speeds for your drive are "up to" 535MB/s Read and 480MB/s Write using ATTO software. Your Reads are above advertised specs but something is wrong with your Writes.
Go to Device Manager. Click on Disk Drives. Right-click on your SSD and select Properties. Select the Policies tab. You should see an option that says "Enable write caching on the device". If it’s unchecked then check it and click "ok". If it’s checked then uncheck it and click ok, then check it and click ok.
Reboot your system and re-run ATTO.
ATTO uses highly compressible data to test Read/Write speeds. AS-SSD & CrystalDiskMark use highly incompressible data to test Read/Write speeds.
AS-SSD & CrystalDiskMark results will always be lower than ATTO.
Done. Same results... Ive never seen any ATTO result like this The write speed reach ~450MB/s at 64 kb data transfer and then fall back to ~250MB/s....
Your result makes sense when you think about it. As your scale up the transfer size in ATTO, the probability that the transfer will need to garbage collect is higher. The OEMs perform those tests on empty drives.
Sorry, don't know what else to suggest. Hopefully other members will get your issue resolved.