Updated: Dec 21, 2022
The fastest xp possible in OSRS is obtainable with the construction skill. With 25 Magic stones, you can create a Demonic Throne, which gives 25,000 xp per throne.
You can ask a butler to fetch magic stones from the bank, allowing you to sit here endlessly like other construction training methods. This gets you 14 million construction xp per hour, but is going to cost you 13 billion gp per hour.
If you wanted 200 milllion construction xp, you could get it in only 15 hours but its going to cost you 185.3 billion gp. If you were to buy bonds, its going to cost you 246,000 US dollars to get to 200 mil.
Demonic thrones are unlocked at level 99 and are very expensive, so the fastest viable construction method is making gnome benches with mahogany planks.
Since you can remove one and start building the next on the same gametick, these are faster than mahogany tables, earning you over 1 million xp per hour.