What a good answer looks like

A person is standing in front of a blank browser tab waiting to be redirected. There is nowhere to hide latency on the read path.

A reader cannot tell when a post was supposed to appear in their feed. So you have seconds of slack on delivery, and almost none on opening the app.

Two people are looking at the same screen expecting the message to appear. There is almost no slack on delivery. There is plenty on everything around it, and the design depends on using it.

Almost every notification has slack. This system's value is that it takes the work off other teams' request paths and spends that slack for them.

A person is standing on a street with a phone in their hand. There is no cooking time and no processing window to hide latency inside. Few problems in this app have less slack.

Almost everything here has generous slack. One operation has none at all, and that operation is the problem.

Uploading and processing have hours of slack. Starting playback has almost none. Designing both paths the same way is the usual way to fail this problem.

A second or two of slack, bounded by something outside your system: the person in the next room shouting at their television.

A hundred milliseconds per keystroke, because a person types the next character in about 200. And hours of slack on how fresh the suggestions are.

No time slack at all, because the decision sits in front of every request. But a lot of accuracy slack, and the design is built on that trade.

Two hundred milliseconds to answer, and the underlying data barely changes. That makes this one of the most cacheable problems on the site, and one of the easiest to over-engineer.

Minutes, sometimes hours, of slack on every output. In this group the usual mistake is failing to use it.

Nobody is waiting for any single page. This group has the most slack in the app, and most good decisions here come from spending it.

The local device must feel instant even with no network at all. Everything else (reaching other devices, agreeing with the server, resolving conflicts) has real slack. The design is built on that asymmetry.

The slack here is not in the request path at all: a caller is waiting on every get and every put. All the slack lives in the background work, and the design succeeds by moving as much as possible into it.

A person is waiting to find out whether their payment worked, and that is the only part with no slack. Everything behind it (capture, ledgers, payouts, reconciliation) has hours, and the design uses those hours to be correct.