I've written 40TB so far to the 1TB 850 Evo in my workstation and it certainly has problems now that it didn't have before - it just hangs for a few seconds at a time, when copying or deleting large number of files. Not sure what it's doing.
If by chance it's an 840 (rather than the VNAND 850), there's been two firmware updates to improve performance over time - make sure to update the firmware.
How full is the drive? If it's more than 80% full you might actually be running into wear leveling issues.