I'm finding this discussion really interesting and it appears to me that it is a case of people doing their very best with an unnecessarily constrained system - be it Metron or whatever...
I have been fortunate to experience ATC providers worldwide - why is it that at KLAX we can be number 3 to land, yet "cleared to land" and not here in Aus? It is obvious to me that (granted that KLAX has MULTIPLE parallel runways and separate arrival and departure runways) they achieve significantly more movements per runway - indeed, on average they achieve a movement per runway per minute (over a whole 24 hour period!), which is significantly higher than the discussed 12 in 30 minutes??
Are our rules "too tight"? Is it simply due to the single runway situation?
Gatwick has a single runway and manages a maximum of 55 per hour.
I enjoy operating in Aus airspace and the service provided by our ATC brethren is directly very good - I'm not trying to agitate, just interested to know where our restrictions come from. Cheers.