Quick Synthesis - Please speed it up!

[Niwashi]

I think we're also forgetting something very obvious in that each synth matches up to the animation, as do many things in this game, and I somehow doubt that SE is going to change it.

But the animation only takes up about half the time of each synth in the batch. Your character will spin their spinning wheel, or make a couple blows with their hammer, or whatever, and then will sit there motionless for a second or two before doing it again. It would impose a limit on how much they could speed it up without changing that animation (or as you mentioned taking another approach like multiple items in the output), but there is some room for speeding it up even without those changes.