At 5th, I think my main "spam" debuff was fear. I can't recall if I had gotten slow yet, but it's really good against individually powerful monsters. Giving up two of my actions for slow that will cost the enemy one of their actions is a good trade, considering I have about three allies who also get three actions each per round. Level 5-6 is also rough for casters, because you're really feeling your proficiency lagging (martials get weapon proficiency increase at 5th and casters get casting proficiency increase at level 7).
This is, however, one area where spontaneous casting is better than prepared casting. If the situation calls for it, I can cast slow round after round, but if I were a wizard I'd probably only prep one. And if we're fighting mooks, well, that's when the fireballs start flying (well, waterballs in my case. And now at higher levels, cones of cold.).
Another meta thing I've noticed is that when you're fighting "bosses", it's fairly common that they're fiends. Fiends often have magic resistance, giving them +2 or so to their already beefy saves, so even spells with decent effects on a successful save often fizzle. But do you know what's good against fiends and undead? Holy water. While holy water only deals 1d6 damage, it also triggers weaknesses against Good damage in fiends which can be substantial. And even a miss deals splash damage.