Samsung has published a fix for the issue that resulted in slow read performance with old data on the company's 840 EVO SSD. It seems voltage drift over time was the key factor that caused the slow read performance but Samsung points out that the issue is unique to the 840 EVO, other TLC-based disks shouldn't be affected by this problem.
The Tech Report tested the Performance Restoration Tool and concludes the tool works as advertised but cautions that we won't know if the EVO is truly fixed until at least a month from now.
Read-retry is used to compensate for changes in cell voltages, suggesting that voltage drift over time was a key factor here. The EVO's flash management routines evidently haven't been responding to voltage changes correctly.
Fixing the problem is a two-part process managed by Samsung's appropriately named Performance Restoration tool. First, the utility flashes the SSD's firmware with the new EXT0CB6Q revision. Then, it reboots the system and completes the restoration process, which we're told involves "conditioning" the NAND. The entire process is non-destructive, so it shouldn't compromise any data on the drive. It also only needs to be run once. That said, the target drive must have at least 10% free capacity for the tool to work its magic.