I don't know if it's already posted, but it's an interesting bug. It's about breaking animation to improve your shooting/casting/whatever speed.
Just hold right click on monster to keep attacking it, and assign some key (I have 's') to stop command. With crossbow - just after shot is fired - press stop key, and you crossbow will fire second shot even faster with bow! With proper timing you can spam crossbow like crazy. It also works with other spells, or even melee dps. For example - dual wield. After two swings and pressing stop button, you immediately attack again - just like it happens sometime when you have auto defend option on - that is, without delay - with talent reducing attack/casting speed - it's even better.
Downside - you can only use it in solo or mirror mode, to make it effective. But imagine, party full of crossbowers. Or dual wielders