I've tested back and forth with a bunch of other phones on the same network and they suffer no issues at all, always instant notifications. I've also messed with every possible setting in the advanced Wifi options, done restarts, tried different channel for my router etc, the lag is there at least 50% of the time.
I should point out this lag is only there when the phones screen is off and in sleep mode. Once the power button is pressed to wake all delayed messages are instantly received. I've also tested and get the same problem with hangouts so it's not a whatsapp issue. I also don't think it's a router issue as my other phones are perfect and my friend is in a different house with completely different router and has the exact thing happening.
Is this a known problem and if so is there any way to solve it? Is there an updated radio that can be downloaded and flashed to the phone without rooting, or do we just have to hope a future update will solve this small but annoying issue?