5B

Good question! So in my experience the thing that bogs down 5e battles is the high number of character abilities. 5B seeks to fix that by pairing those down to something more similar to what you see in an OSR style game. However, because 5B is compatible with 5e we kept the same HP and Damage thresholds for PCs and Monsters.
So I think you will find combat quicker than 5e but perhaps not as quick as B/X.
If you're looking for an easy OSR style game with very quick play I would recommend Cairn by Yochai Gal, The Black Hack by David Black, or Knave by Ben Milton.

Your first guess is correct.
I must have rewritten these a half dozen times during play testing to make them clearer, but the format always got in my way. Maybe for a future release I’ll just replace all of them with a general lvl 1-20 chart.

I've actually been waiting to see if somebody noticed this. It was one of the big things that I missed in editing before release (I even put the Dragon up on the main page just to really tempt fate).
You're not missing anything, creatures shouldn't have a weakness mod of 0, that's just a mistake on my part from when I was converting stuff out of the SRD. For now, you should just assume at least a -1 but more if you think it makes sense.
This is one of the things that are going to be fixed in my next update. I've just been gathering some more feedback for other stuff stuff to change before finishing my edits of the PDFs.
Excellent review by the way. Very helpful.

Thanks for the helpful reply! One more thing: are MUs really unable to use any weapon at all? Not even a staff? And if the answer is no, they can't use weapons, can they punch something?

They are not "skilled" in the use of any weapons (or armor for that matter). As demonstrated in...one of the examples. You can still use anything you want but you don't get to add your Skill Bonus (or receive its bonus in the case of armor).
You can always punch things, but I didn't think unarmed damage was important enough to include. You could always use 5e's rule for it I suppose (1+Str Mod).

I'm sorry for missing that! In case it helps as you edit the rules, I think my confusion came from having played a lot of BX but comparatively little 5e. So, in my mind, there's a (BX) rule against MUs even *wearing* armor, and that's what made me read "no armor" as "you may not wear armor" and not just "you can wear any armor but don't get the bonus." And that in turn made me think that "no weapons" meant "you may not use weapons." All of which was my mistake, of course -- you're right about the example. (I will have to get used to the idea that MUs can use swords and such -- I'm more used to the rule that they may only use a dagger!)

Thanks so much! And yes! A creature book and module are already under way.

Heck yeah, Tower Silveraxe with 5B! The OSR module I play tested with was Willowby Hall.
Conversion from OSR to 5B is a good question, not really something Harald and I designed for but I can talk to him about it. Maybe we can put together a doc on that and Monsters at some point.

I had a pretty easy time just using monsters out of the back of the book straight, as is, without too much trouble. The difference in what gold is expected to level up is the only killer! When you can possibly get a couple 1,000 platinum and gold coins from clearing out a dungeon you run into the problem of your PCs being able to buy everything they'll ever need; and possibly jumping several levels at once.

The treasure rules are set up so that it takes about 3 hoards to level up, so if you need to convert an OSR module it's easiest to figure out how many levels you expect the players to gain. What I sometimes do is do a quick tally of all the treasure in the module (I just grab the large treasure rooms and then multiply it by 1.5 or 2 depending on how much small loot is in the dungeon) then look at the XP requirements for the system that dungeon is for (these vary surprisingly!) and now I know how many levels worth of loot is in that dungeon. Then I use the 5B experience tables to figure out how much "new" loot that is.

Say there's 15000 gp in a dungeon meant for lvl 3 characters, and the system requires 3000 xp to level between 3-4. That means we got enough xp to level up 5 characters. In 5B, those 5 levels would need 9000 gold (2700-900 times 5). So now we know that the difference is a factor 0.6. So each treasure they find I roughly halve its value.

An easier way is to just...See more ballpark it, a party should gain a level every 2-3 sessions, so they should find a hoard worth of gold/xp every session. You know how many rooms they'll get through (my players typically do 5 or so), so just make sure there's one hoard worth of stuff to find in the next 5 rooms :).

Oh wow, I hadn't realized how bad they got after I made them searchable and bookmarked. I can definitely upload an unoptimized hirez version until I can fix that compression issue.

Thank you so much!

Thanks for the kind words! Spell damage is the flat number in parentheses next to the indicated power tier. Our math has that number map to the average damage a full caster in 5e can do in that level range. So like, instead of rolling damage dice, the spellcasting check is kind of already doing that for you with higher rolls not only allowing for higher power tiers but also higher damage output.

For me, this is a literal game-changer. This doesn't intimidate a new player like a pile of $40-a-piece books and as a DM it's everything you need in one small booklet. My DriveThru account has over 140 titles I've tried to switch to or make a simple system out of. None of them has hit the mark like this one. Thanks for doing all the hard work cause now I get to go back to enjoy playing again!

I’m glad it will save you a bunch of work. Bringing the OSR streamlined game experience to 5e was a big design goal for me. It’s what a wanted for my home games so I built this.
And thanks for the nice review!