At T5 my strategy is always to arrive at the gate (this is assuming the flight is on time) slightly less than 10 minutes before the 'gate closes' time on the boarding pass. I find this means most people have already boarded and you are also 'protected' from them closing the flight without you on board.
However, exceptions to this are
- domestics when overhead space is at a premium
- longhaul from T5B where they normally have a seperate line for Club, First and Silver/Gold which works pretty well.
However I take the OP's point that T5 is a bit of a nightmare for a first-timer, but as soon as you learn how to use it properly it becomes very efficient indeed. But you need to learn!