A Vitest suite can drop to less than half of its run time after one boring find-and-replace.

Finding the cause

Vitest can report how long each import takes:

// vitest.config.ts
export default defineConfig({
  test: {
    experimental: { importDurations: true },
  },
});

The profile can be surprising: the vast majority of test time is spent importing modules, not running tests. The top offender is often the @mui/icons-material barrel. The package has ~2000 icon modules, and

import { Edit, Delete } from "@mui/icons-material";

loads the whole index. Bundlers tree-shake it for production builds, but the test runner transforms and evaluates it, in every test file that touches such a component. That can cost tens of seconds per affected file.

The fix

Use per-icon paths:

import EditIcon from "@mui/icons-material/Edit";
import DeleteIcon from "@mui/icons-material/Delete";

Most of the codebase already did this; only some files still use the barrel. Convert those and ban the barrel with ESLint so it can’t come back:

// eslint config
rules: {
  "no-restricted-imports": ["error", {
    paths: [{
      name: "@mui/icons-material",
      message: "Import icons by path: @mui/icons-material/<IconName>",
    }],
  }],
},

Takeaways

• If tests are slow, profile imports before optimising tests.
• Barrel files (index.ts re-exporting everything) are cheap for bundlers and expensive for test runners and dev servers.
• When you fix a convention, add a lint rule in the same PR, or it will regress.