XP boosts from the store are the only legitimate accelerator that meaningfully changes the total. A duration boost multiplies the XP from every game for a set number of days, which suits concentrated play; a per-win boost suits scattered sessions. Used well, they shave perhaps a quarter off the total.
ARAM is frequently recommended as faster. It is not faster per hour — the XP rate is broadly comparable — but games are shorter, so it feels quicker and it is less mentally taxing. If you are levelling while doing something else, ARAM is the more sustainable grind.
Co-op versus AI is slower than it looks. It pays reduced XP specifically to discourage bot levelling, so what you gain in game length you lose in rate.