App ideas cost nothing. Every one of them looks shippable in a weekend, and then the weekend turns into three months of auth flows, sync bugs, review notes and a paywall that took longer than the app.

So before Xcode or Android Studio opens, put the idea through four questions and write the answers down. Who pays? What problem does it solve? Can you say what it does in one sentence? Why would anyone switch, and what stops Apple from copying it? Ask them in that order. Money goes first because it kills weak ideas fastest, while a kill still costs an evening.

Every app idea has a boring 90% and a nasty 10%. The boring part is CRUD, auth, a settings page, Stripe. The nasty part is the one subsystem that decides whether the thing works at all, and it’s usually not the part that looks hard in the pitch. Below are nine builds worth attempting, with the nasty 10% named up front so you can decide whether you want to spend your weekends there.

A team spends three weeks rewriting a hot loop in Rust bindings, shaves forty milliseconds off a function that runs once per request, and ships it. The page still takes four seconds to load. Nobody profiled first. That’s the pattern behind most performance work that goes nowhere: real effort, wrong target.

Optimization only pays off when you know where the time actually goes, and that’s rarely where intuition points. Developers tend to suspect their own code, because that’s the part they wrote and the part they can picture running slowly. The database, the network, the third-party API call buried in a middleware layer — those stay invisible until you measure them.