I too have the same issue on the stock rom . I found switching over to a kitkat rom helps as it locks on within seconds. I've tried madhi rom and Houstonn PA roms. Unfortnately, I am enjoying the Stock rom LG G2 features at the moment, particularly the knock on, LG Quick Remote and LG Quick window features of stock.
Kernel version 3.4.0
Software Version D803R10c