They stated that a party fight should last 30-40 seconds depending on gear. So when designing monster spawns they should take the number of enemies in a givin area into account.

"May be a bit too much work to spot check everywhere so a slight global respawn increase would be nice it's simply far too long as is"

But in areas they are custom tailoring for grinds they can use that figure and compile it with the ammount of viable monsters in said area, and extrapolate a fair respawn timer from there.