KLAX does not average 1min per movement per runway during its best hour let alone consistently. You may be confused with the stat one departure and one arrival per minute (on all four runways). Segregated arrival and departure runways is actually the most ineffiecent way of operating a multi runway airport. This is because gaps need to be put in for wake turbulence that a departure could fit into. So you lose 4 or 5 slots per hour. But it is however noise friendly and much much easier to manage from an ATC perspective. Especially with four runways; mixed mode at LAX would be very complex.
20 arrivals in half an hour into a single runway operation a quiet day? Well certainly quiet for departures because there wouldn't have been a single one; that would equate to arrival spacing of around 2.5nm. The most efficient mode is one arrival, then 1 departure. The theoretical minimum is 2 minutes between arrivals (30 per hour {4NM gaps}) and a departure in the gap (total of 60 per hour). Remembering that this is a perfect world scenario with 0 margin for error. Single runway rates across Australia are running arrival rates between 28-30 per hour for total movement rates of 55+
Melbourne 16 only is particularly painful at the moment because they need to allow 3mins between arrivals (20per hour {7NM gaps}) to allow the arrival to roll through. Bearing in mind that this also slows the departure rate down to 3 mins. So now you're only moving 40 per hour. This doesn't sound like a lot but it makes a HUGE impact on delays when you're at capacity.
In any case don't be comparing Melbourne with airports like Heathrow and Los Angeles, that's a bit rediculous. They're nothing alike. I feel your beef needs to be with the CASA runway nomination criteria. It really should allow for 10kts tailwind and 30kts crosswind. Seeing places like Sydney grind to a halt because of a 25kt crosswind ruins the flow across whole country, and seems a bit conservative.