What should I invest in my Elemental Codex?

[Grimoire-M]

Spell Speed's effect on DoT damage was never worth investing in. If it was then most DPS classes would've attempted to use it in HW. It provides a percentage damage buff to DoTs, but it's weaker than what you would've gotten from Crit or Det, even without the oGCD and TP/MP factors pushing those further ahead. BLM was the only class that stacked it at all. It would be better for DoTs if it had the same effect as Haste in WoW, where it increases the DoT tick rate, thereby increasing the potency (though it would also be nice if it affected ability cooldown rate too).

[Zyneste]

Spell Speed's effect on DoT damage was never worth investing in. If it was then most DPS classes would've attempted to use it in HW. It provides a percentage damage buff to DoTs, but it's weaker than what you would've gotten from Crit or Det, even without the oGCD and TP/MP factors pushing those further ahead. BLM was the only class that stacked it at all. It would be better for DoTs if it had the same effect as Haste in WoW, where it increases the DoT tick rate, thereby increasing the potency (though it would also be nice if it affected ability cooldown rate too).

It was heavily invested in by DPS oriented SCHs. With 4 Dots and a weaker filler spell, spell speed affected SCH the most aside from BLM.