next.js build times are slow. How can I make them faster?

Question

I've investigated both the Next JS documentation as well as similar questions like Slow page build time in development with Next.js and TypeScript (which is TypeScript specific - this question concerns JavaScript and does not involve compiling TypeScript)

I am using next.js 10.0.9 and after running next, my app takes around 50 seconds to build and begin responding to HTTP requests.

After making a change, it takes another 12 seconds to rebuild. This seems much slower compared to other popular JS frameworks.

More detail:

• npm run dev simply runs next and next takes around 50 seconds to become responsive (just after the compiled successfully is printed.

This means tasks like git bisect to find where a bug was introduced are very slow, as next has to do a full 1 minute rebuild after checking out each commit.

$ npm run dev

> [email protected] dev /home/mike/Code/myapp/alpha/frontend
> next

ready - started server on 0.0.0.0:3000, url: http://localhost:3000
info  - Loaded env from /home/mike/Code/myapp/alpha/frontend/.env.local
info  - Loaded env from /home/mike/Code/myapp/alpha/frontend/.env.development
info  - Loaded env from /home/mike/Code/myapp/alpha/frontend/.env
warn  - React 17.0.1 or newer will be required to leverage all of the upcoming features in Next.js 11. Read more: https://err.sh/next.js/react-version
Warning: Built-in CSS support is being disabled due to custom CSS configuration being detected.
See here for more info: https://err.sh/next.js/built-in-css-disabled

info  - Using external babel configuration from /home/mike/Code/myapp/alpha/frontend/babel.config.json
event - compiled successfully
event - build page: /
wait  - compiling...
event - compiled successfully

• Making small changes to files requires around 12 seconds of compiling... watching the triangle before the page becomes responsive at compiled successfully.

How can I speed up next.js build times?

Updates

Running @next/bundle-analyzer helped - thanks @juliomalves - I can see we're loading all of react-heroicons (that's the big index.js in the picture) which ~I can easily fix with more specific imports.~ Update - this is now done though out the codebase 👍

Old:

> import * as Icon from "@graywolfai/react-heroicons";

New:

import { BellOutline } from "@graywolfai/react-heroicons";

This has speed up my build times (after running next, the compiling task finishes faster). However my bundles still seem to be the same size.

Answer 1

This question is a few months old, but I'd recommend upgrading Next.js.

The most recent versions of Next.js (11, and 12) have large performance improvements to start-up time, hot reloading, and compilation. And they tend to be very backward-compatible.

Version 11 blog post: https://nextjs.org/blog/next-11

Version 12 blog post: https://nextjs.org/blog/next-12

Answer 2

I've faced the same issue with next.js version 12.1.5.

My problem was that I was forgetting the server npm run dev was still running - after closing npm run dev and running the build again the build completed in almost 3 minutes.