@enfors Remember, thinking a thing doesn't make it so.
I know the process of putting things together and when things need to be done under stress. You are never going to be looking up monsters that are close to one another in level under stress. That's just not when it happens.
Either it's going to be while you're doing prep, in which case you have hours, or it's going to be while the PCs are doing things and you're putting something together for the next random encounter, assuming you haven't intelligently already put together random encounter tables.
Which brings up random encounters, by the way, and the fact that random encounter tables include the name of a creature and not its stats. Anytime you go to resolve a random encounter, you don't need to search by level, you're searching by the name of the thing. You want that to be the quickest, easiest, lowest friction activity.
This is just a matter of looking at the workflow and seeking the point of highest friction.
I'm not saying that an index of creatures sorted by level isn't useful, because obviously there are uses for it. It's just not the primary use of a list of creatures.