what is more fun with GURPS fast magic or slow magic?

[simply Nathan]

I'm most fond of one-second casting times in GURPS and one-round casting times in D&D. I like wizards to be able to do something magical every turn if need be; they're going to tire themselves out faster than the thieves and warriors if they do so anyway.

If I see it lined up that some slower spellcasting method is in play for "flavor" reasons, that usually means that the intended flavor of the campaign will not be to my liking; if it's for "balance" reasons that can only mean that I wouldn't have any fun as a spellcaster even if the flavor and tone otherwise suited me.

I have cast all of three spells in combat in GURPS, one of them in a practice session our GM had us go through to get an understanding of the combat rules. Those spells were fireball (for 9d burn against a cave bear, lighting it on fire despite its stone-like skin), fireball (8d into a field of tall, dry grass to deter some rat-sized ants that were attacking us), and fireball (1d against a velociraptor; the GM ruled that I could cast and throw a 1d missile spell as a single-turn action in that campaign).