Yea i dont believe that Slow spell should be the end all be all solution to countering Haste, there should be other means to inflict Slow and Slow-like conditions too. I'm thinking something along the line of Slow-poisoned arrows, Slow bombs like a counterpart to Haste Spore bombs. As for non-humanoids, maybe make some creatures have a passive where their blood is so foul-smelling that if you hit them in melee range and get some of that blood splattered onto you, you lose the ability to perform additional actions. Just to name a couple of ideas.

Originally Posted by ZOZO1006
you will be suprised how few spells have they prepared.
Agreed. I think the ideal solution would be to give enemy spellcasters a wider range of spells to work with, especially crowd control and utility spells.
I'm no programmer but I imagine that would require lots of improving their AIs to be able to effectively use those spells, which would be very labor-intensive assuming its even possible.

As for Haste itself, i just looked up the spell in DnD (never played DnD myself), and i think that version of the spell is pretty fair, not sure why Larian felt the need to flatout buff it.