Measure CPU startup time locally

To measure the startup time of your app, run the CPU profiler with following command in your terminal:

This command builds your app and runs the generated bundle code once. It then watches for changes and rebuilds the app when necessary.

Every time you save a file, an updated startup time measure is logged to the terminal. You can use this information to observe trends when making changes in your app.

The CPU profiler displays your app's startup time in the terminal. It also generates a more comprehensive startup time report and saves it to ./startup.cpuprofile.

To view this report, open it with a flame graph viewer such as Flame Chart Visualizer for JavaScript Profiles in Visual Studio Code, or Speedscope in a browser.

After you find the parts of your app that contribute to the startup time, you can start optimizing them. Some common scenarios that can cause slow startup times include the following:

• Using dependencies that perform unnecessary computations during import. For example, a dependency might establish a connection with a third-party API during import.
• Calling functions or template literals that perform computation outside of a request cycle, such as at the top-level of a module. For example, a common scenario is using graphql-tag to parse GraphQL queries at the top level of a route file, which internally transforms strings into AST objects. This can be avoided by using a #graphql comment instead, which turns on syntax highlighting for GraphQL queries without parsing them.