I've just installed the 5.99.99 firmware on my 42 LB650V (UK) and i can confirm there is a definite improvement in web OS speed for me, Q.menu is now actually reasonably quick, still not quite instant but definitely an vast improvement over the previous 4.4 whatever it was, also a lot faster at navigating around my external HDD.
I'll add the link to the site but I can't take credit for it
direct link to 5.99.99 firmware
Installation instructions (USB must be FAT32)
The root directory of the USB-drive, create a folder "LG_DTV" and copy the firmware file. Turn on the TV, insert a USB flash drive. You will see a message "open in smart ball", select "No", and then will be prompted to upgrade. The installation progress can be seen in the Settings-General-Information on the TV. Do not turn off the TV or remove the USB flash drive until the end of the upgrade process.