The compiler is indifferent to your variable names.
Give it valid syntax and it executes without complaint — x or userSessionTimeout, makes no difference. The CPU doesn't know what a good function name looks like. The runtime doesn't care that processData could mean a hundred different things in a hundred different files.
The people who follow you into the codebase care enormously.
Code runs in milliseconds. Engineers read it for years — to understand what it does, extend it without breaking it, debug it under pressure, review it before it ships. On any codebase with a working team behind it, reading events outnumber execution events by an order of magnitude. The machine is the least demanding audience the code will ever have.
This asymmetry changes what good code actually means.
A three-line conditional can collapse to one line with a ternary. Identical output, half the space, twice the cognitive load. The compute saved is immeasurable. The time cost moves from the machine to every reader who follows.
d and daysSinceLastLogin execute identically. One of them tells the next engineer what the code is for, who it tracks, and what breaks if the value is wrong — in the time it takes to read the word. Ten extra characters. An order-of-magnitude difference in comprehension time.
Functions named processData and handleStuff are broad enough to describe every function in the codebase and precise enough to describe none of them. The engineer debugging at 2am pays for that ambiguity.
Legibility is load-bearing. It determines how fast a team moves, how safely they refactor, whether a new engineer contributes in days or weeks. Clever code that only its author can maintain has a carrying cost that compounds until someone rewrites it.
A comment explaining what the code does is evidence the code needs to be rewritten. A comment explaining why is a gift.
Short functions are a constraint that forces a decision: either this function does one thing, or it gets split. Both outcomes are better than the alternative.
A variable name that could mean two different things to two different readers is the wrong name.
Write for the human who opens the file with no context, under pressure, at the worst possible moment.
The machine will handle itself.