This is something I have just noticed and ended up testing and the results are relatively astonishing.
(It might also be an unintended bug caused by certain spells' effects.)
Long story short, using a AOE effect spells that don't have DC such as Obscuring Mist seems to speed up the spawn rate & time of wave-based event during quests.
There are a few caveats from what I could test out in the various quests in Korthos Island, but the speed-up is quite real.
(I have yet to test it out for other quests further down though so take this with a grain of salt.)
What seems to happens is that spells like Obscuring Mist trigger a "negative" effect on most mobs without actually be countered by DC's, hence it affect both mobs that are visible and invisible mobs that have yet to be set active, but have, from the game engine perspective, spawned.
For example, during the Cannith Crystal quest, using Obscuring Mist in a position that allow the effect to reach multiple "spawn" point will trigger 2-3 enemies to become visible and active at the same time (instead of 1 at a time with a slight timer specifics ones).
During the Collaborator quest, the initial waves of enemies almost spawn immediately if you cast Obscuring Mist before talking to Jacoby.
(The whole waves before the behind-the-bar chest spawns spawns in barely 6 seconds if you're able to clear the enemies fast enough.) There's a bit of a timer before the last wave (after opening the behind-the-bar chest), but the whole initial bar fight barely last 10s TOTAL while Obscuring mist is active. And I noticed that I only have to cast it around the entry point where Jacoby comes it.
During Redemption, the wave-based end part of the quest, the Sahuagins almost pop-out instantly instead of having their "spawn & jump in" slow beginning.
The caveats previously mentioned are:
1) Since all monster spawn really fast, defending quest can become MUCH harder as you end up fight a bunch of enemies that shouldn't be brought together as fast.
(For example, with the Cannith Crystal quest, if you don't clear up the enemies fast enough, they can easily rush to the Crystal and destroy it almost immediately. You need to properly controls aggro during defending quests.)
2) This trick cannot skip pre-determined timers. For example, if there's a timer between waves (unseen), it will still be active.
3) This trick affects reduce to a fraction of a sec the initial phase when enemies spawns. Some have some sort of animation and comes out of hiding and stuff like that and this trick cut down that visual phenomenon to nothing and enemies simply pop-up instantly.
4) Some are also spawning, but are kept hidden until their "phase" becomes active and (which is why I think this might be an unintended bug for now), spell like Obscuring Mist seems to trigger their phase as soon as they are instantiated. (Hence, you can end up with 2-3 waves at once depending on how waves are managed by the game's engine pre-battle.)
5) Friendly NPCs will also act a lot faster toward enemies as hidden/stealth enemies seems to comes out of stealth almost instantly while affected by the Mist's effect. (Friendly NPCs normally ignore inactive enemies that are not yet active and only attack hidden enemies that are about to be come visible. Enemies affected by Obscuring Mist seems to be instantly visible to Friendly NPCs and trigger hostility from such friendly NPCs right away.)
(It might also be an unintended bug caused by certain spells' effects.)
Long story short, using a AOE effect spells that don't have DC such as Obscuring Mist seems to speed up the spawn rate & time of wave-based event during quests.
There are a few caveats from what I could test out in the various quests in Korthos Island, but the speed-up is quite real.
(I have yet to test it out for other quests further down though so take this with a grain of salt.)
What seems to happens is that spells like Obscuring Mist trigger a "negative" effect on most mobs without actually be countered by DC's, hence it affect both mobs that are visible and invisible mobs that have yet to be set active, but have, from the game engine perspective, spawned.
For example, during the Cannith Crystal quest, using Obscuring Mist in a position that allow the effect to reach multiple "spawn" point will trigger 2-3 enemies to become visible and active at the same time (instead of 1 at a time with a slight timer specifics ones).
During the Collaborator quest, the initial waves of enemies almost spawn immediately if you cast Obscuring Mist before talking to Jacoby.
(The whole waves before the behind-the-bar chest spawns spawns in barely 6 seconds if you're able to clear the enemies fast enough.) There's a bit of a timer before the last wave (after opening the behind-the-bar chest), but the whole initial bar fight barely last 10s TOTAL while Obscuring mist is active. And I noticed that I only have to cast it around the entry point where Jacoby comes it.
During Redemption, the wave-based end part of the quest, the Sahuagins almost pop-out instantly instead of having their "spawn & jump in" slow beginning.
The caveats previously mentioned are:
1) Since all monster spawn really fast, defending quest can become MUCH harder as you end up fight a bunch of enemies that shouldn't be brought together as fast.
(For example, with the Cannith Crystal quest, if you don't clear up the enemies fast enough, they can easily rush to the Crystal and destroy it almost immediately. You need to properly controls aggro during defending quests.)
2) This trick cannot skip pre-determined timers. For example, if there's a timer between waves (unseen), it will still be active.
3) This trick affects reduce to a fraction of a sec the initial phase when enemies spawns. Some have some sort of animation and comes out of hiding and stuff like that and this trick cut down that visual phenomenon to nothing and enemies simply pop-up instantly.
4) Some are also spawning, but are kept hidden until their "phase" becomes active and (which is why I think this might be an unintended bug for now), spell like Obscuring Mist seems to trigger their phase as soon as they are instantiated. (Hence, you can end up with 2-3 waves at once depending on how waves are managed by the game's engine pre-battle.)
5) Friendly NPCs will also act a lot faster toward enemies as hidden/stealth enemies seems to comes out of stealth almost instantly while affected by the Mist's effect. (Friendly NPCs normally ignore inactive enemies that are not yet active and only attack hidden enemies that are about to be come visible. Enemies affected by Obscuring Mist seems to be instantly visible to Friendly NPCs and trigger hostility from such friendly NPCs right away.)
