As a WHM I focus on Crit > Det > SSpeed. In short, the only time I meld speed is when there's no slot for the other two, which is rare. Given 99% of content is extremely telegraphed and pre-casting isn't exactly difficult due to the long cast-times of enemy abilities, focusing on Spell Speed would only be a benefit in moments where you're on the back foot for some sporadic reason, and even in those moments most Healers have some sort of backup tool to remedy most of it, typically off the GCD, so having spell-speed is of little benefit in any half-decent group. Having said that, the only other beneficial stat would be Piety, which is even more iffy.

Crit is beneficial for both healing and damage, even if the payoff is RNG. Det is a more consistent/flat benefit for damage so is always welcome. There's no need for Piety as most have an answer for MP, or run with groups that container refresh-capable classes.