Subsystems and You 10: Simplified Spellcasting
Though they don’t get as much spellcasting ammunition as their spontaneous counterparts, even prepared casting classes eventually get to the point where they have dozens of spell slots to work with.
Now, an experienced player can no doubt hash out the individual spells they wish to prepare each day fairly quickly, particularly with classes that have a spellbook or otherwise limited selection of spells, though with enough experience even prepared divine casters that have full access to their entire spell list can pick and choose what they want very quickly, especially if either example knows what they are likely to face each day.
However, maybe you’re indecisive, or you’re inexperienced with the class, or maybe you’re just afraid of dragging the game down. Personally, I’d get GM permission to begin preparing as soon as it is confirmed that the night has passed with no further incident, but that’s just me.
For those that legitimately want a faster way of doing spells, we have the Simplified Spellcasting optional rules from Pathfinder Unchained, which only focus on the highest levels of spells that the mystic has access to.
Needless to say, this option only applies to prepared casters, since all spontaneous casters draw from a predetermined group of known spells anyway. Interestingly, however, something similar to this actually cropped up again in the way that Starfinder handles spellcasting NPCs, essentially not listing their lower-level spells as they are unlikely to be used by a foe that is not likely to survive the encounter, or at least not stick around after their strongest magics have been used up.
In any case, this system works for any full or mid-power prepared casters (minor 1-4 spellcasters don’t gain anything from this system, really), and once the spellcaster gains access to 4th-level spells, they gain a pool of general spell slots based on their level, key ability score, and bonus slots from class features with which to spontaneously cast spells below their top three spell levels, which are calculated normally.
This does mean that spellcasters using this system will inevitably have less total spell slots than those using the core systems, but the advantage is that they need no longer prepare lower level spells ahead of time, and can be assumed to just have the right spell on tap, at least until those lower level slots run out.
Of course, this is less useful if your build includes spells at lower level that retain their usefulness even into high levels, where you might want to have full access to your total spell slots.
Personally, I’m not a super big fan of this system, but I’m sure that there are those out there that would get a lot out of using it, even if it’s not personally something I would use.