Real user monitoring vs synthetic testing

Synthetic testing and real user monitoring are often presented as competitors. They are not. They are complementary tools that answer different questions. Synthetic testing tells you whether a page can be fast. Real user monitoring tells you whether a page is fast for your actual users. You need both.

What synthetic testing is good for

Synthetic tests run a consistent, repeatable script against your page from a known location, on a known device, on a known network. They are perfect for catching regressions during development, before they reach production. They are also useful for testing specific user flows, like checkout, that may not generate enough RUM traffic to be statistically meaningful.

The weakness of synthetic tests is that they do not reflect real user experience. A synthetic test from a data center in Virginia on a cable connection does not represent a user on a mid-range Android phone in Brazil on 3G. If you only look at synthetic data, you will optimize for the wrong users.

What real user monitoring is good for

RUM captures actual user experience, on actual devices, on actual networks, in actual locations. It tells you the truth about how your page performs for the people who matter. RUM is the only way to see that mobile LCP in Brazil is 4x slower than in the United States, or that Safari users see a 200ms slower INP than Chrome users.

The weakness of RUM is that it only tells you about production. It cannot catch a regression before it ships. It also cannot tell you why a regression happened, only that it did. To debug a RUM regression, you usually need to reproduce it synthetically.

The combined workflow

Use synthetic tests in CI to catch regressions before they ship. Use RUM in production to catch the regressions that synthetic tests missed. When a RUM regression appears, use synthetic tests to reproduce and debug it. When a synthetic test reveals a regression, use RUM to confirm the fix in production.

// Synthetic test in CI
import { test } from '@playwright/test';
import { expect } from '@pagepulse/synthetic';

test('homepage LCP under 2.5s', async ({ page }) => {
  await page.goto('/');
  const { lcp } = await expect(page).webVitals();
  expect(lcp).toBeLessThan(2500);
});

PagePulse supports both modes. The same SDK captures RUM data in production and synthetic data in CI. Both feed into the same dashboards, the same budgets, and the same alerts, so you always have a complete picture of your performance.