Another thing I changed in my incremental revisions to GURPS Magic for my DF game is the casting times for Mass Daze, Mass Sleep, and Mass Suggestion.
Right now, Mass Sleep, for example, costs 3, minimum 6, time to cast is 1 second per point of energy. Therefore it takes 6 seconds to cast Mass Sleep on a 3-hex diameter area (aka area 2.) Want to hit a whole combat spread across 5 hexes? That's area 3, 9 seconds. Diameter 7? That's area 4, 12 seconds.
12 seconds is a lifetime in GURPS combats, and it's a resisted spell, and not cheap (12 points is more than a normal human's entirely allotment of Fatigue Points), so it's not even a fight-ender unless you roll very well and/or your foes fail badly. That's long enough that it's better to just Great Haste several of your friends or hit the area with Stench or Create Fire.
I'd suggest time to cast is the same as the minimum energy cost.
Mass Daze: 4 seconds
Mass Sleep: 6 seconds
Mass Suggestion: 8 seconds
None of them are especially fast, but combined with the revisions to the spells I suggested yesterday, this makes them much more viable. Mass Daze can take out bunches of foes long enough to go for a Telegraphic All-Out Attack to a vital spot on each, and Mass Sleep can put many opponents out completely. Serious casters will get skill 20+ and halve the time, too.
That makes them viable offensive spells. Plus, generally big spells are more expensive, not slower. There are a few exceptions (Windstorm is one), but they more than make up for it with fairly extreme effects.
We have been using these rules for a while, in case anyone was wondering why Mr. NPC Wizard got off such a big spell so quickly in my DF game.
Sharp-eyed folks will notice there are four Mass spells in GURPS Magic - the last being Mass Zombie. I didn't mention it above because I feel like it's an exception. It's meant not as a combat spell, but as a big, slow spell meant for waking up the dead on a battlefield or in a cemetery, and raising a (temporary) army of zombies to beat on your foes. It's not a combat spell, it just has to compete with Zombie, not things like Create Fire or Rain of Stones.