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Vibe Coding · レッスン

パフォーマンスとコスト

高速なアプリと健全なAI/API利用料金を実現します。

「パフォーマンスとコスト」はCoddyKit上の無料Vibe Codingレッスンです。 これはレッスン4/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはVibe Coding学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Vibe Codingコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Fast Apps, Sane Bills

Your app works — but is it fast, and what does it cost to run? Vibe-coded apps often have hidden performance problems and surprise AI/API bills. This lesson shows you how to keep both under control.

You don't need deep optimization skills. You need a few habits and the right questions to ask your AI.

Measure Before You Optimize

The biggest mistake is guessing where the slowness is. Measure first. A quick timer around suspicious code tells you the truth — often the bottleneck isn't where you'd expect.

This snippet times how long a piece of work takes.

function slowWork() {
  let total = 0;
  for (let i = 0; i < 1000000; i++) total += i;
  return total;
}

const start = performance.now();
slowWork();
const ms = performance.now() - start;
console.log('Took ' + ms.toFixed(2) + ' ms');

The Most Common Slowdown: Doing Work in a Loop

A classic performance killer is calling a database or API inside a loop — one call per item, hundreds of times. The fix is usually to fetch everything in one go.

Spot the difference.

// SLOW: one database call per user (the N+1 problem)
for (const id of userIds) {
  const user = await db.getUser(id);
  process(user);
}

// FAST: one call for all of them
const users = await db.getUsers(userIds);
users.forEach(process);

Ask AI to Find Bottlenecks

You can hand your code to AI and ask it to hunt for the slow parts. Be specific about what you suspect so the answer is focused.

Review this code for performance problems. Look especially for:
- database or API calls inside loops
- the same data fetched repeatedly
- work that could be cached
Rank the issues by impact and show the fix for the worst one. Here is the code: <paste>

Cache What Doesn't Change

If you compute or fetch the same thing over and over, save the result and reuse it. This is caching. A simple in-memory cache can turn a slow repeated call into an instant lookup.

const cache = new Map();

function getExchangeRate(currency) {
  if (cache.has(currency)) return cache.get(currency); // instant
  const rate = expensiveLookup(currency);             // slow, runs once
  cache.set(currency, rate);
  return rate;
}

function expensiveLookup(c) { return c === 'EUR' ? 1.08 : 1; }
console.log(getExchangeRate('EUR'), getExchangeRate('EUR'));

Don't Ship a Giant Frontend

On the web, performance is often about how much you send to the browser. Huge images and bloated JavaScript bundles make pages load slowly, especially on phones.

Ask your AI tool to slim things down — it knows the modern tricks.

My web page loads slowly. Help me reduce what's sent to the browser:
- compress and lazy-load images
- split the JavaScript so unused code isn't loaded upfront
- point out any large dependencies I could drop
Tell me the single change with the biggest impact first.

AI Calls Cost Real Money

If your app calls an AI model (like the Claude or OpenAI API), every request costs money based on tokens — roughly, the amount of text in and out. A popular app making many calls can run up a real bill fast.

The instinct to send huge prompts "just in case" is what blows budgets. Send only what's needed.

Cut AI Costs Without Cutting Quality

Several levers reduce AI spend dramatically:

  • Use a smaller/cheaper model for simple tasks; save the powerful one for hard ones.
  • Shorten prompts — trim filler and only include relevant context.
  • Cache answers for repeated questions instead of re-asking the model.
  • Limit the output length when you don't need a long reply.
My app calls an AI API on every user message and the bill is high. Help me cut cost:
- which requests could use a cheaper, smaller model?
- how do I trim the prompt without losing accuracy?
- can I cache repeated questions?
- how do I cap the response length safely?

Set Budgets and Alerts

The simplest way to avoid a shocking bill is to set a spending limit and an alert. Most API dashboards (Anthropic, OpenAI, your hosting provider) let you cap usage or get notified at a threshold.

Turn this on before you launch, not after the bill arrives.

Don't Over-Optimize Too Early

Balance matters. Spending days shaving milliseconds off an app with ten users is wasted effort. The rule: make it work, make it correct, then make it fast — and only where it actually matters.

Measure, fix the biggest real problem, and move on. Premature optimization makes code complex for no benefit.

A Performance & Cost Checklist

Before you ship, run through this with your AI:

  • Measured the slow parts — not guessed.
  • No database/API calls stuck inside loops.
  • Repeated expensive work is cached.
  • Frontend ships compressed images and split JS.
  • AI calls use the right-sized model and trimmed prompts.
  • Spending limits and alerts are set.

Quick Check

Your app feels slow. Before changing any code, what's the smartest first step?

Recap: Fast and Affordable

You learned to keep vibe-coded apps fast and cheap: measure before optimizing, avoid calls inside loops, cache repeated work, slim down the frontend, and control AI/API costs with smaller models, trimmed prompts, caching, and spending alerts.

And don't over-optimize — fix what matters, where it matters. That wraps up Productionizing AI Code: your apps are now tested, secure, maintainable, fast, and affordable. Real builder territory!

よくある質問

「パフォーマンスとコスト」レッスンは無料ですか？

はい。「パフォーマンスとコスト」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Vibe Codingコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Vibe Codingコースには全4レッスンが含まれています。

「パフォーマンスとコスト」で何を学びますか？

高速なアプリと健全なAI/API利用料金を実現します。 ブラウザで直接実行するハンズオンコードでVibe Codingを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Vibe Codingを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのVibe Codingは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン4/4です。

「パフォーマンスとコスト」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このVibe Codingレッスンでコードを書いて実行できますか？

はい。すべてのVibe Codingレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. AIでテストを追加する
  2. AIコードのセキュリティ基礎
  3. 保守しやすいコードを保つ
  4. パフォーマンスとコスト
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