You don't think there's a throughput limit for mortals? Some threshold that can't be exceeded without destroying the mortal?
Well, yes and no. There's certainly a limit to what a person can handle all at once. But we've seen through thaumaturgy that a wizard taking his time and preparing carefully can well exceed those limits. This is probably also something that scales with a wizard's age, personal power and experience.
Let's for the sake of argument say that a spell's energy can be measured in Thaums as a unit.
An apprentice might be able to throughput only 1 or 2 thaums at a time, and might take a few minutes to do each. While any spell is theoretically doable, something that's 100 thaums would end up being an arduous marathon task, effectively putting it out of his normal reach.
Someone like Harry, a full wizard with some experience, might be able to do 5 thaums/minute, making that 100-thaum spell difficult, but but a 20 minute task.
Say someone on Ebenezer can do 20 every 30 seconds, putting the 100-thaum spell in at only 2 minutes 30 seconds. At this point, he'd be able to do a 1,000-thaum spell with the time and effort it takes Harry to do the 100-thaum spell.
And so on.