Samsung SSD 840 speed issue
I have also experienced slow performance on an SSD and it was due to other active processes writing to the disk continously.

Have a look at question on superuser on how to monitor OSX disk activity: https://superuser.com/questions/89266/whats-causing-all-the-disk-activity-on-os-x

Basically run this either this command form Terminal:

iotop -C 5 12


or this other:

fs_usage


To locate any process that is writing to the disk continously.

Another thing to try is booting from a clean install of OSX on an external USB disk, and the benchmark the internal SSD.

I would also determine if the SDD is running the latest Samsung firmware. You can download the latest version from this Samsung support page: http://www.samsung.com/global/business/semiconductor/samsungssd/downloads.html

Beware that it is recommended to have a backup of the data before attempting to update the firmware.