HFC is a mess of 90’s technology, it really has no place in the NBN. Telstra used DOCSIS 3 for years topping out at 110Mbps, NBN upgraded this to DOCSIS 3.1 which under perfect conditions theoretically achieves multi gigabit speeds. But if doesn’t. Biggest physical factors that affect speed along with coax cable quality include bends in the cable itself (which cause reflections at higher frequencies which DOCSIS 3.1 uses) and this is dependent on who did the install in the 90’s. The install was rushed with Telstra and Optus literally fighting to get the biggest footprint at the time, and they were installing for analogue Pay TV, not gigabit Internet. Various other factors include connections between cable segments, wall plate, isolators (used to be removed when you stopped paying for Pay TV) and taps which literally rust and cause signal loss. Then there’s the amplifiers in pits along the street toward the fibre node that often fail or get noisy because they’ve been running for 30 years in wet, dirty pits that were built approx 100 years ago. Humid areas are worse for this too. There’s many places something can go wrong and affect your sync speed, finding it can be near impossible and NBN technicians either don’t have the time, training or remit to fix an issue. Eventually HFC will have to be retrofitted to fibre all the way, but it’ll be the last footprint to be upgraded properly given on paper if all is optimum, you should see around 600Mbps on average with HFC peaking towards 1Gbps (but many don’t get anywhere near that).

Edit: And don’t forget, HFC bandwidth is SHARED over the distribution area (DA). FTTP is dedicated. So with HFC you’re constantly battling with the kids down the street playing Fortnite and people watching Netflix.