Duff’s device in JavaScript raced hand-written loops that copy one array into another. A reader asked the follow-up: how do they compare with Array#copyWithin, the built-in that copies a range inside a single array? That is a different workload, so it needs its own measurement. The short answer: on every Node and Deno we can install today, the built-in runs 47 to 84 times slower than the loop.

“Premature optimization is the root of all evil” — our field’s favorite half-sentence, quoted far more often than the sentence it was cut from. Usually it serves as permission: build it fast, profile later, fix a thing or two, done. Let’s put the sentence back together and ask what it licenses: when is optimization premature, and when is “premature” the excuse?

In 1983, Tom Duff needed to copy memory into an output register faster than his compiler could manage, and wrote the most famous abuse of switch in the history of C. I ported his device to JavaScript and raced it against the plainest possible loop — and the verdict changed with the engine, the engine’s version, and the CPU underneath.

Most of the numbers that decide an architecture can be settled on the back of a napkin, before a line of the prototype is written. A handful of formulas and a latency ladder you can recite from memory rule out bad designs a profiler could only catch after they’re built. The same numbers expose the quieter traps — like a redundant trio of replicas that one dead node takes down whole. Let’s do the arithmetic.

I once needed the SLA for an endpoint my dashboard leaned on, so I asked the team that owned it. Their lead came back with 200ms ± 500ms. Read that literally and the fastest responses arrive 300ms before the request is even sent. The number wasn’t malicious — it came straight out of the standard formulas. The formulas were wrong for the data, and that mistake is everywhere.

Great programmers cheat. A hard problem gets quietly swapped for an easier one; a transaction-grade database is replaced by a flat file nobody misses; machinery everyone else considers mandatory simply never gets built. They know a lot — and that’s exactly why they get away with it.