First question: what date are you travelling. Things will be different if it is after the initial T4 Long Haul move.

Based on my experiences in the current state of T5:

Arrive HEX, lifts up to departures. Turn right when in the main departure area. You want the South end of the building. This is where you have the F desks. Also, South security tends to be quicker than North, although I've tried both and never waited over 10 minutes. This is including the BP 35 minute check.

The Flounge is also at the South end of the building.

Once you've cleared South Security turn left, walk some 100 yards and you will have Down Escalators. Walk 150 yards and you will have Up escalators to The South Lounge Complex. Flounge is directly to your left after the first set of escalators.

On average when arriving from HEX I've been in the lounge within 15 minutes.

Enjoy it.

If you travel Post LH T4 move 1, I guess things will be less smooth.