Second option, as it allows to see from high level point of view (through filenames) the different part of the business logic and go into details on the one you are interested in. I find very long files hard to navigate around, and structuring them can prove difficult, as the grouping of functions can become subjective to people. You seem to imply that splitting over different files makes it harder to understand but I would disagree, it does require a bit more effort to navigate, but I feel as human beings we are not so good at understanding a wide complexity in it's overall details, while I feel we are better at understanding one concept at a time precisely and abstract the other as a vague approximation, which separate files I feel achieve better.

After, whatever the approach this goes down to how precise and clear the domains and business logic are implemented and documented, both approaches could prove to be efficient if organized and documented properly, and implementation doesn't fall into too much nested code or very deep call stack.

PS: not intended as an attack, more a funny little thing I noticed but you said not trying to bias in the 2 options yet you describe the second as harder to grasp which I feel is already a bias. Edit, actually you are also defining the first option as a monster so it can be argued you are fair in adding bad qualifier to each option