For details of the performance problem: anandtech.com/show/8550/samsung-acknowledges-the-ssd-840-evo-read-performance-bug-fix-is-on-the-way
anandtech.com/show/8617/samsung-releases-firmware-update-to-fix-the-ssd-840-evo-read-performance-bug
We find the latest BIOS and instructions here: samsung.com/global/business/semiconductor/minisite/SSD/global/html/support/downloads.html