What are the advantages of bows vs crossbows in runescape?

I've written this up on a phone as I'm fairly busy, so please excuse poor formatting and english. If anybody feels like cleaning this up feel free.

The weapon you choose post November 2012 in RuneScape generally doesn't matter as long as they are in the same tier (eg. Level 50 two-handed sword vs Level 50 two-handed spear). The main difference between weapons is two-handed vs dual wielding (unless you want to use a shield for whatever circumstantial reason). Dual wielding will ultimately provide the same or similar maximum potential damage as a two-handed weapon, but the two-handed weapon usually takes longer between hits and if you miss, then you miss entirely, whereas with dual wielding you have two chances to hit.

I may have made it out to sound like dual wielding is just objectively better since you have twice the chance to hit, but this is not true for things like PvP where having a much larger single hit is very important for the final blow.

Example, a barbarian has 110 / 150 hp (depending on which barbarian). Most of the times i will be able to one shot the barbarian but for the sake of the question lets say that i dont. I hit him for 90 hp. This leaves him with 20 hp. Looking at the damage stats of both weapons i know both weapons will kill him (if i dont miss) would it be wiser to now switch to my shortbow instead of waiting for the (slower) 2h crossbow?

Yes, but the game runs on a 600 Ms tick, so it probably isn't worth risking the attempt to swap weapons and ammunition in PvE because if you perform the switch late into the tick, then you'll have to wait a minimum of two ticks before firing at the enemy again.

What determines how fast a weapon fires?

An arbitrary number decided by Jagex for that specific weapon based on multiplication of 600 Ms.

If the weapon says its an avarage speed. Avarage compared to?

Average can pretty much just be replaced with medium in this instance because it isn't average in the sense that Jagex took the speed of all weapons and found the average.

The weapon speed listings are as follows:

• Slowest: 7.2 seconds/12 ticks at 600 Ms.
• Slow: 4.2 seconds/7 ticks at 600 Ms.
• Average: 3.6 seconds/6 ticks at 600 Ms.
• Fast: 3 seconds/5 ticks at 600 Ms.
• Fastest: 2.4 Seconds/4 ticks at 600 Ms.

Sources:

• Playing the game
• Reverse engineering
• The RuneScape Wiki