First, I wanted to preface my post by saying, Yes: spellspeed is bad on red mage.

However, there's a huge number of people who are completely misunderstanding spellspeed itself.

Spellspeed reduces the cast time and the recast time of every spell by a percentage.

So if you have enough spell speed to reduce those by 5%, then the 2s cast time is becoming a 1.9s cast. The 2.5s recast is becoming a 2.375s recast. The true value of spell speed is increasing your damage by increasing the number of casts you can get off across the course of the fight. The cast time of the first spell is irrelevant to this, because it's shorter than the recast. The fact that the second spell is instant cast is irrelevant, because its recast is being reduced.

Now, having that out of the way: Why, then, is spellspeed so bad for red mage? Simple. Because a very very high percentage of rdm's kit is completely unaffected by it. Corps a Corps, melee combos (excepting the last two gcds), fleche, contre sixte. All of these care not one whit about spell speed, and they make up a lot of our damage. Now, that's not to say Spell Speed is useless, it's just the least impactful stat on red mage by a good margin.

Black mage loves spell speed for the same reason: it increases the number of casts they can get. It may be unique in that they're increasing the number of casts by reducing the longer cast time, but at the end it has the same net effect.

EDIT: Ah, it seems someone else got to it before me