The main reason behind the read performance degradation can be found in cell charge decay over time, which is caused by extensive read-retry cycles. The new fix should bring periodic refresh for old data and which recovers cell charge back to its original state.
While there are some worries that the new fix could impact user performance, Samsung noted that it won't be the case as it periodic refreshing is a relatively low priority process that will only run when the drive is idling. Users will be able to update their 840 EVO firmware via Magician 4.6, which also includes an Advanced Performance Optimization feature.
PC Perspective already had a chance to try out the new firmware and it appears that it mostly fixes the issue without running the optimization tool.
Of course, we can't be sure that the new fix works and if the read performance will stay high after certain amount of time as the problem was never with freshly written data.
While Samsung had a lot of issues with read performance on its EVO 840 SSD, it is good to know that the company is constantly working to get it fixed and hopefully this time it will be a permanent fix.