How can I improve function cold start performance on Vercel?

Vercel Functions enable developers to write server-side logic to connect to their database or external APIs. When a function starts for the first time, it’s a "cold start". Subsequent requests to that function are then considered warm.

This guide will help you improve the performance of your functions and understand how to determine if the latency increase is from a cold start. With our latest improvements to functions, like powering them with Rust, bytecode caching, and automatic warming for paid plans, most customers will not see cold starts for production traffic.

The following suggestions will help you ensure optimal performance of your Vercel Serverless Functions:

1. Choose the correct region for your functions: Functions are deployed to US East by default. You can change the default region for functions in your project settings. Choose a region that’s closest to your data source for optimal performance.
2. Choose smaller dependencies inside your functions: Cold start times are correlated to function size, which is often mostly from external dependencies. If you have large dependencies, parsing and evaluating JavaScript code can take seconds or longer. Review your bundle and try to eliminate larger dependencies using a bundle analyzer.
3. Use proper caching headers: Function responses can be cached using Cache-Control headers. This will help ensure optimal performance for repeat visitors, and Vercel’s Edge Network cache even supports stale-while-revalidate headers. Note that cache misses will still need to request data from your origin (e.g. database) rather than reading directly from the cache (faster).
4. Upgrade CPU or memory for your functions: Ensure your Vercel Functions are using the "Standard" performance option, the default on Pro plans. This can reduce startup time. Learn more.
5. Update to the latest version of Next.js: The latest version of Next.js (v14.2+) includes significant startup performance improvements, especially if using external packages like Sentry.
6. If using Pages Router, bundle external dependencies: Setting bundlePagesExternals to true under the experimental flag is strongly recommended for customers using Pages Router, and will result in a large reduction in cold start seconds.
7. Use dynamic imports: While an uncommon use case, customers with sites using divergent code paths, using dynamic imports to load only the necessary code can reduce cold start times.