Most of what max1 says goes for PH as well, without the noise sharing (thankfully), the parallel rwy and MAESTRO.
Wouldn't these be RPT with a known schedule.
PH, I think because we don't have MAESTRO, has slot times. All RPT operators submit schedules the night before, a sequence is worked out and landing times sent back to the companies. Some days it works better than others, some days it doesn't work at all. Unfortunately, we never go back and look at the days it doesn't to see if anything can be learned.
It could work miracles if it were allowed to. RTA + - 5 seconds, efficient descents, the whole bit. Why would the 52 other RNAV equiped aircraft be 'doing their own thing'?
A few issues. If you are on the same route as traffic immediately ahead/behind in the sequence, separation becomes an issue. For example, two jets, same route, same distance. #1 issued with a FF of x, calculates profile cruise, slow descent. #2 issued a FF of x+2, calculates profile cruise, slower descent. Now the top jet can't have descent until the distance opens up to 5 miles, the distance won't open until there is a speed difference, and there won't be a speed difference until they are on descent.
Another, for PH anyway, is sequencing prior to radar. If the flow waits until all the traffic is on radar to calculate the sequence, you will get significant delays at only ~120-140 miles out, so the sequence is now done further out and based on pilot estimates. (As an aside, PH ATC cannot see ADS-B.) Pilot estimates only have to be revised if they differ by 3 min or more. It's not unknown now for us to issue a fix time the aircraft can't meet because its now
earlier than possible.
Additionally a lot of aircraft struggle to get +/- 1 minute, particularly if the winds are bad. In a sequence of 20, even an average of 5 seconds late adds up, and sod's law says the early ones never balance out the late ones.
I can say however a controller palming me to another frequency for further descent as I'm about to capture an altitude at high speed is very very annoying and is very uncomfortable for everyone on board. It seems also enroute to Perth from the east coast that the clearance seems to arrive a bit later than it should and we end up all arms and legs when its not what we expect.
Is that assigned A100 from ML Centre and then call PH APP for further? It is poor technique, but sometimes its quicker to tell you to call rather than coord a lower level with PH and then give it to you. What should happen of course is that you are transferred to APP before you need further descent. I was taught that arriving aircraft should never have to ask for descent. I have to confess I never knew leveling out on descent was a workload issue, I'll bear it in mind. We really do have very little understanding of how things that affect pilots unfortunately.
With regard to speed reductions and tolerances, we only see a radar calculated groundspeed rounded of to the nearest 10 knots. The speeds we issue tend to be 'suck it and see' because we issue indicated speed to control goundspeed. If you are doing 255 IAS instead of 250 we wouldn't know. If you are still catching, we'll slow you down more.