steg1947, I wouldn't put it down to Throttling as you have suggested, but to congestion and being 2.5km distance from the tower. Having looked at Cronulla on the Optus Coverage map, there are not that many towers to cater for the population in that area. Admittedly I don't know how many antennas are on each tower.
Also looking at the RFNSA website https://www.rfnsa.com.au/ that gives more details of what Telco's are transmitting from those towers (3G, 4G, 4G+, 5G) and there actual location. There are 2 towers transmitting 5G within the Cronulla area, which the website provides the actual location.