The most efficient way to use Presence of Mind is still using it for DPS spells, in any situation.

There is just no damage as of now that requires PoM for burst healing (even Almagest from O4S, currently the hardest-hitting raid buster in the game can easily be handled without it), and using it for a Raise not only has very little impact (less than 2s cut), but you will also waste half of the duration of a 150s cooldown on a single spell.

Lightspeed has 3 main functions, which IMO follow the priorities below:
1 - Situations where you need to AOE/heal while moving. Having Helios as an instant cast spell is incredibly useful
2 - Cutting the MP cost for the spells you're using
3 - Quick burst healing

So you could say AST is a more mobile healer, but WHM's Cure III is still unbeatable when it comes to AOE healing output - in fact it's so strong that it isn't needed outside of a few specific fights.

As for using Lightspeed for Ascend, I'd recommend only doing that in an emergency, as like with PoM you will be wasting a lot of the duration on a single spell.