A little Spell Speed makes no noticeable difference, while tons of it not only do little for your primary heal CDs but are completely unnecessary given the way combat works in this game.

No amount of spell speed is going to fix poor life choices (whether by the healer or the party). At best high SPSD will give reactive players more room for error, but healers who know their fights don't benefit much at all.

E.g. You know when Prey is about to go out if you pay attention to the fight in T10. The only thing more SPSD gives you is a fraction of a second longer to dislodge your thumb from your ass and attend to the mechanic that shouldn't have been any surprise to begin with.