For ventures, the 1hour ventures are best(not the actual Quick Ventures mind you, which are RNG and most of the time give shit xp). But only if you can do x a day, where x is well, the amount you get from 18h / the amount you get from 1h. You can see the values so you can do the math, iirc it was somewhere like 6 a day at low lvl, but goes down to like 3 a day at 50+, so at that point unless you don't play a whole lot, it's much better to spam 1hour ventures. If you play at regular hours you can also combine both by simply doing 3-4 1h ventures, and then doing a 18hour which will be done by the time you log in the next day.
Only other way is buying a level boost for retainers in the cash shop, obviously. I'm thinking of buying one once I figure out what class I'll be leveling next after Dancer for my combat retainer, since it's unlikely to be Monk(my current retainer class) and I want that retainer leveling and gathering shit for my crafting at some point. I've relvled my retainers not long ago cause they were in the wrong order(and obviously in true FFXIV fashion there's no way to move them around probably because of bad code) and this shit took me like a full month doing 6-10 1h ventures a day which is crazy slow.
