Weird, weird issue this one, have tried everything I can possibly think of. Most benchmark results report at least roughly in the correct ballpark, (although sequential write is about 60% of what it should be) except for the 4k write results and random write IOPS, which are shockingly low. Have only had the drive about 6 months.
Brand new install of Windows 8.1 Pro x64
Gigabyte XA-990FX-UD3 Motherboard
Samsung 840 240GB drive
Have installed motherboard chipset drivers, am on the latest bios version of the board, bios confirms it's in AHCI mode as it should be.
I've run CrystalDiskMark and Samsung's Magician to confirm this too. Have gone through all of Samsung's optimisation options, have turned off write caching, disk indexing, System Restore, Windows Search service and all the other usual candidates.
Compared to this (which looks like ideal results admittedly, and on the Intel AHCI controller rather than the AMD one, but the disparity is a bit...!)
Samsung magician reckons:
And now I've re-run Crystal disk for screenshot purposes it's reporting something a bit more sensible...