Int affects Mana Consumption and Casting Time

I believe I actually know why it costs 20 mp instead of 25 mp now to cast fireball. Back before they decrease casting times, it took 25 mp and 5 seconds for one charge of fireball, which is 5 mp per second. After they adjusted it to 4 seconds for one charge, they neglected to change the mp drain rate while charging, causing it to become 5 mp per second for 4 seconds, or 20 mp now. Of course, this is just a theory.

In that regard, perhaps all intelligence is doing is just decreasing casting times while not affecting the mp drain rate during casting, causing the overall cost of the magic spell to go down.