Updated: Dec 21, 2022
The fastest herblore xp method involves 1-ticking potions that have stackable ingredients. The highest level potion that you can 1-tick is extended antifires, which require lava scale shards mixed with regular antifires.
1-ticking these is a little tricky since you have to move your mouse a lot more than the other methods, but doing this efficiently gets you almost 900k herblore xp per hour.
I wouldn’t do this method since theres so much clicking, so the fastest viable method out there is making ancient brews.
Making ancient brews gets you about 520k herblore xp per hour.