The 840 uses TLC NAND just like the Ultra II does. When it comes to maximum performance, maximum durability, and the best error correction in a single package, that SSD will be either MLC-based or eMLC-based SSD. While MLC SSDs tend to give better real-world speeds, the bigger things to note is that TLC-based drives, as a general rule, have very inferior endurance and they encounter several issues with error corrections that MLC and SLC drives have much fewer issues handling. Makers are now producing tons of TLC SSDs and TLC NAND has gotten much better, but it is still very inferior to MLC IMHO. They do this because TLC costs far less than premium MLC. The full tour-de-force of SSDs IMO is one with Power Loss Protection, which is always a nice feature to have given loss of power while a SSD is writing can brick it.
Were you using TRIM with the 840? The Samsung drives seem to need TRIM more so than the SanDisk & Crucial/Micron drives that use Marvell controllers. Have you tried plugging the Samsung into a SATA port that has power but not data - and leave it for a day or so and then check to see if the SSD’s CPU cleaned up the drive? (hence giving you good speeds back?) Did Samsung Magician help?
Also, have you pulled SMART data on the drive? TLC NAND has a relatively low endurance and, if you are using your SSD for intenses usage with tons of write cycles, your issue may in fact relate to the service life of the drive coming to a close.
Provided the 840 isn’t sick, you should be getting better performance. I’m not sure what your model should get, but I have a Samsung 850 Pro inside my 2011 MBP-15 and an 840 Pro inside my MBP-13. Both have written several hundred gigabytes, and SMART data shows that the SSDs are still very healthy. Both had read and write speeds right around 500 MB/s, and to this day they will still attain those speeds (I ise the third party TRIM control built into OS X, I enabled TRIM when the drive was new.)
