There seems to be two different ways that people speedrun The Binding of Isaac: Rebirth. The first is to run a race where you need to beat the boss in the chest with Isaac. The second is where you beat the game sequentially with a set of the characters. All the ones from the original game, or all the ones from this game, or all the ones from this game excluding the crazy challenge character. I've watched a fair number of both kinds of runs and one thing they have in common is the player will check out the treasure in the first item room and reset if it isn't very good.
I've just been playing with all the items and I find I can win most of the time, but sometimes it is definitely very slow going. It certainly makes sense to invest some time at the start of the run into getting a high damage item. But what are the high damage items? I decided to take a look at the list of items in the item room pool and see which ones look like they'd be good to start with. Next time I play I'm going to try resetting until I get one of them and try to get a feel for how long it takes. I also need to make sure I've unlocked all the really good ones! If not then I should make getting them a high priority.
The way I see it, 35 of the 220 possible item room items are really good. There are also 4 items in the curse room that are super good (ceremonial robes, the mark, the pact, and pentagram) and it seems like it's worth starting with any item room item if you get one of those. Anyway, the 35 items that I want to try starting with are: 20/20, blood of the martyr, capricorn, chemical peel, chocolate milk, cricket's head, dark bum, death's touch, dr fetus, epic fetus, ipecac, iron bar, judas' shadow, lil' brimstone, magic mushroom, the mind, mom's knife, money = power, monstro's lung, mutant spider, number one, odd mushroom (large), phd, pisces, polyphemus, proptosis, pyromaniac, rotten baby, smb super fan, stigmata, tech.5, halo, inner eye, ludovic technique, and sad onion.
I actually have all but 3 of those items already. I need to beat the boss rush with Judas (should be doable), I need to kill Satan with The Lost (I don't even have The Lost unlocked and it would be super hard regardless), and I need to kill Mom's foot with the activated ability of The Bible. I should be able to do that last one by just keeping my eye out for The Bible to show up in a store. While playing Judas!