I have been building a mod to enhance combat in Skyrim. The main basis of some parts of the mod is a hidden perk that acts like an ability, with an "Apply Combat Hit Spell" entry. Every time the player character strikes something, a scripted effect is run and this may invoke other special effects.

Just a quick example:

Hidden Perk Effect: Whenever the player character strikes something, apply a script affect to the recipient NPC.
Script Effect:
If SomeCondition, move an activator to the recipient NPC and apply a lightning blast effect on it.
else if SomeOtherCondition, move an activator to the recipient NPC and apply a health drain effect.
else if YetAnotherCondition, move an activator to the recipient NPC and script a whirlwind around it that knocks down nearby foes.

Effects are applied using standard activator casting functions.

One problem I found: There's some awful delay somewhere with the casting function. In practice, the effects are applied about a second after they should be. In practice, this is unacceptable; causing AoE effects to miss and generally being an inconvenience.

Applying effects using the player character as a source also produces similar delays.

Is there any "cast instant" or "apply effect instant" function that has none of these delays?

Are those conditions determine in the script? If so, then that's may be what is causing the delay (hard to tell without the actual script there...). Papyrus can be somewhat slow depending on the functions being used, the objects you call the functions on, and so on. I would try making multiple magic effects, and then use condition functions within those magic effects to determine which one gets applied.