The SSD 840 EVO is a victim of problems that Samsung intends to correct through the deployment of a new firmware.
If the SSD from Samsung emerges as genuine references, they are not without problems. As proof, the owners of the 840 EVO model will be offered a new firmware that can be applied via the Magician software for Windows or disk image, after obviously backing up their data first.
Stamped as EXT0CB6Q, this new firmware is supposed to improve reading performance with the oldest data. It in fact would correct an error in the algorithm update of the status of cells, which would result too aggressive in terms of process and would impact performance in reading for older data.
In short, you can go without a prior concern. It only remains to wait for the availability of the final version.