Running a large number of tests in an Nx workspace with many Angular libraries can indeed be slow. To speed up your tests by 10-20%, you can try the following strategies:

Parallelization: Run tests in parallel by utilizing the computing power of multiple CPU cores. You can use the --parallel flag or set the maxParallel option in the Nx configuration. Example: nx run-many --target=test --all --parallel or update nx.json with "maxParallel": <number_of_parallel_tasks>.

Affected tests only: Instead of running all tests, run only the tests affected by your changes using nx affected:test. This will significantly reduce the number of tests executed and speed up the process.

Test batching: Group your tests into smaller batches and run them concurrently. You can use Jest's --maxWorkers flag to control the number of workers running tests. Example: nx test my-lib --maxWorkers=4.

Caching: Use Nx's built-in caching mechanism to cache the results of previous test runs. This can speed up your test execution, especially in CI/CD environments. You can enable caching by updating the nx.json file with the following configuration:

json
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"tasksRunnerOptions": {
  "default": {
    "runner": "@nrwl/nx-cloud",
    "options": {
      "cacheableOperations": ["build", "test", "lint", "e2e"],
      "accessToken": "your-nx-cloud-access-token"
    }
  }
}
Jest configuration optimization: You can update your Jest configuration to optimize test runs. For example, you can set the bail option to stop the test suite after a certain number of failures, or use the --changedSince flag to run tests only for the code that has changed since a particular commit.

Test debounce: If you're running tests in watch mode, you can use the --debounce flag to control the delay between file changes and test runs. This can help avoid running tests too frequently, especially when making a series of small changes. Example: nx test my-lib --watch --debounce=500.

Code optimization: Optimize your code and tests by using more efficient algorithms, reducing unnecessary dependencies between components, and minimizing the use of heavy computations or network requests.

Environment-based test splitting: If you have a CI/CD environment, you can split your test suite across multiple build agents to reduce the overall execution time. This can be achieved by using tools like jest-junit and the CI environment's parallelization features.

Remember that the results of these strategies may vary based on your specific setup and environment. It's essential to experiment and measure the improvements to find the best combination of techniques for your project.