I don't think your figures here are correct - perhaps you're citing the minimums rather than the maximums. Max spells known for 18 Int was 18, and I think for 17 Int may have been 14 (I haven't got my PHB here, but this is the number given in a letter to the Forum in Dragon #122).A character with a highly respectable 16 Intelligence had a 35% chance to be unable to understand a given spell and was limited to 7 per level. With a 17, his chance to be unable to understand a spell was 25% and he got at most 8 per level.
The time requirements in 1e were, let us say, tougher than in any later edition. Now, I don't have my books on me, but I am sure I will be corrected if I misspeak: Our magic-user needs 8 hours of uninterrupted rest followed by 1 hour of uninterrupted study in order to regain his single spell.
Billd91, you pre-empted my reply to RC.The spellcaster needs a certain amount of rest to regain spells based on the highest level of spells he wants to regain. It starts at 4 hours for 1st and 2nd level spells, 6 hours for 3-4th level, 8 hours for 5-6th levels, 10 hours for 7-8th, and 12 whopping hours for 9th level spells.
And that's not all. Each spell level to be recovered requires 15 minutes. For first level spells, that's 15 minutes each. Second level spells 30 minutes each, etc.
I can't find anything about interruptions but I think I've looked long enough to make the point that recovering spells in 1e takes a LOT longer than 3e. Fire off a bunch of spells and you'll be spending substantial amounts of time recovering them.
But I don't entirely agree with your last paragraph. As a 4th level wizard in 1st ed AD&D I have 3 first and 2 second level spells. At 5th level I get a new 3rd level spell. So time to rest and memorise all spells at 4th level is 4 hours + 7*15 minutes is a bit less than 6 hours. I think that's less than 3E, which I believe for arcane casters requires 8 hours (though I think RC is right that it's less harsh as far as interruption is concerned). At 5th level it's 6 hours + 10*15 minutes which is eight-and-a-half hours. That's not very much more than 3E. I can't remember the spell table past this level - but even supposing that 6th level adds both a 2nd and a 3rd level spell, that's still a little less than 10 hours, which isn't hugely more onerous than 3E.
And once we get above 6th level, we're getting into the zone where 1st ed MUs are pretty strong - multiple fireballs or lightning bolts, stinking clouds or webs, and multi-target magic missiles, and often a wand or two to use in combat as well. Plus the utility options available. In my experience 7th+ level AD&D MUs were pretty potent characters regardless of the % chance to learn mechanics.