It can't be said that there's one single factor causing delays.
Maintenance required may be one, down from the occasional collision or accident (not an A333/332 but B789 VH-ZNC recently) to tiny areas such as pitot tubes. ATC-initiated delays may be another, such as Japanese airports not having an available slot if QFi services run badly late, as could be flight or cabin crew unavailability.
Disruptive passengers or delays with those connecting from unpunctual domestic flights to international departures might contribute. Weather such as lightning strikes doesn't help. Slow unloading or loading of catering/ULDs for baggage and freight/mail may be factors.
Medical incidents on board thankfully don't occur every trip, but a diversion can add three hours plus to a flight.
I've only scratched the surface.
However your general point is valid that it stands to reason as aircraft fleets age, maintenance becomes more time consuming or difficult. Gradually despite stocks once held, some spare parts may take longer to source if urgently required.
tomleee1986, I'm sure you've seen occasional social media posts showing various interiors of QFi/QFd planes held together with tape. There was one I saw that showed a (toilet?) door with tape.
