add a changelog entry about Open Next perf improvements

[vicb]

Summary

fixes https://jira.cfdata.org/browse/DEVX-1983

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/changelog/	@cloudflare/pm-changelogs, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://20722d36.preview.developers.cloudflare.com
Preview Branch URL: https://vicb-open-next-perf.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/changelog/2025-06-05-open-next-size/	https://vicb-open-next-perf.preview.developers.cloudflare.com/changelog/2025-06-05-open-next-size/

[irvinebroque]

These are the internal details of what we did

But ultimately at the end of the day, what people care about is:

better performance via lower startup time for their worker

is there a before/after comparison we could do for a starter app?

i.e. - startup time before with OpenNext was X milliseconds, and now it is Y milliseconds?

[vicb]

@irvinebroque

These are the internal details of what we did [...]

What about changing

- We [removed `babel` from the app code](https://github.com/opennextjs/opennextjs-cloudflare/pull/710) to save ~4MB on a typical uncompressed app size
- We were also able [to drop a dependency on `@ampproject/toolbox-optimizer`](https://github.com/opennextjs/opennextjs-cloudflare/pull/700) to save ~3MB on a typical uncompressed app size
- We [stopped preloading all the app routes](https://github.com/opennextjs/opennextjs-cloudflare/pull/696) to save >100ms on the first request.

for:

The generated bundle size for a Next app created via create-next-app was decreased from 14 to 8MiB (2.3 to 1.6MiB gzipped).

The startup time for a medium-sized Next app would typically be 100ms faster.

Note that the startup time used to depend on the number of routes in the applications.
That's why I wrote "medium-sized" and "typically".
It's vague but I don't have a better idea.

Feel free to propose something now that you have the numbers.

Thanks!

[lrapoport-cf]

@vicb one small note but otherwise lgtm :)