It took them a while to get it right, but Samsung did manage to fix their read degradation issue in many of their TLC equipped 840 Series SSDs. I say many because there were some models left out when firmware EXT0DB6Q was rolled out via Magician 4.6. The big exception was the mSATA variant of the 840 EVO, which was essentially the same SSD just in a more compact form. This omission was rather confusing as the previous update was applicable to both the 2.5" and mSATA form factors simultaneously.
The Magician 4.7 release notes included a bullet for Advanced Performance Optimization support on the 840 EVO mSATA model, but it took Samsung some time to push out the firmware update that enabled this possibility. We know from our previous testing that the Advanced Performance Optimization feature was included with other changes that enabled reads from 'stale' data at full speeds, compensating for the natural voltage drift of flash cell voltages representing the stored data.
Now that the firmware has been made available (it came out early this week but was initially throttled), I was able to apply it to our 840 EVO 1TB mSATA sample without issue, and could perform the Advanced Performance Optimization and observe the expected effects, but my sample was recently used for some testing and did not have data old enough to show a solid improvement with the firmware applied *and before* running the Optimization. Luckily, an Overclock.net forum member was able to perform just that test on his 840 EVO 500GB mSATA model:
Kudos to that member for being keen enough to re-run his test just after the update.
It looks like the only consumer 840 TLC model left to fix is the original 840 SSD (not 840 EVO, just 840). This was the initial model launched that was pure TLC flash with no SLC TurboWrite cache capability. We hope to see this model patched in the near future. There were also some enterprise units that used the same planar 19nm TLC flash, but I fear Samsung may not be updating those as most workloads seen by those drives would constantly refresh the flash and not give it a chance to become stale and suffer from slowing read speeds. The newer and faster V-NAND equipped models (850 / 950 Series) have never been susceptible to this issue.