Performance Testing

1 The Hook

It was 9:00 AM on a Tuesday in March. A major NZ bank had just launched a new home loan application portal. The marketing team sent an email to 80,000 customers. Within 15 minutes, the site was down. Not hacked. Not buggy. Just overwhelmed.

The bank lost an estimated $2.3 million in application volume that morning. The root cause? Nobody had tested what happened when more than 200 people used the site at once. The database connection pool maxed out. Threads hung. CPU spiked to 100%. Customers saw spinning wheels and timeout errors.

A single afternoon of load testing would have caught this. But performance testing is often treated as an afterthought — something you do "if there is time." In enterprise software, that mindset is expensive.

In NZ, performance testing is especially critical for:

• Banking and finance: End-of-month payroll, tax season (IRD integrations), mortgage rate changes
• Government: Census submissions, benefit applications, visa processing portals
• Retail: Boxing Day sales, Click Frenzy, new product drops
• Utilities: Power company billing cycles, water restriction announcements

2 The Rule

The key word is defined. "Make it fast" is not a requirement. "The 95th percentile response time for the checkout API must be under 800ms with 5,000 concurrent users" is. Performance testing turns subjective feelings ("it feels slow") into objective data.

3 The Analogy

4 Types of Performance Testing

Performance testing is an umbrella term. Each subtype answers a different question. A comprehensive strategy uses all of them.

5 Key Metrics You Must Measure

If you cannot measure it, you cannot improve it. Here are the metrics every performance test report should include:

Percentiles matter more than averages. If your average response time is 400ms but your p99 is 12 seconds, 1% of your users are having a terrible experience. In a system with 100,000 daily users, that is 1,000 frustrated people.

6 Tools in Action

Each tool has strengths. The right choice depends on your tech stack, team skills, and budget.

import http from 'k6/http';
import { check, sleep } from 'k6';

export const options = {
  stages: [
    { duration: '2m', target: 100 },   // ramp up
    { duration: '5m', target: 100 },   // steady state
    { duration: '2m', target: 400 },   // stress
    { duration: '5m', target: 400 },   // sustained stress
    { duration: '2m', target: 0 },     // ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<800'],  // 95% under 800ms
    http_req_failed: ['rate<0.01'],     // error rate under 1%
  },
};

export default function () {
  const res = http.post('https://shop.example.co.nz/api/checkout', {
    product_id: '12345',
    quantity: '2',
    region: 'auckland',
  });

  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time < 800ms': (r) => r.timings.duration < 800,
    'has order_id': (r) => r.json('order_id') !== undefined,
  });

  sleep(1);
}

7 When to Use It / When NOT to Use It

8 Common Mistakes

9 Now You Try

10 Self-Check

Click each question to reveal the answer.