Haste and spell casting - how does it work?

[LineageRazor]

Probably by removing the need to chant anything, so the user can channel the spell right away.

This would be my guess. To force-cast the spell without any incantation takes significant mental focus, so it can only be done periodically. Certain other spells are simple enough that they can be canted instantly without the mental buildup, like Aero and Scathe, but the tradeoff is that they are not very powerful.