This is my frontend tip of the day.
If you have a frontend that consumes an external API:
1) Retrieve the json responses from devtools
2) Save them in your project as json files (trim the data a bit if it's too long)
3) When starting your app with a special env var like MOCK_DATA, make your app mock the data and use your saved json data instead.

You can associate each response with an url regex.

The package fetch-mock mocks fetch really well, it lets through the urls that don't match anything.
This way you can incrementally add responses.

And voila, you have a mode where you have near instant page loads to test things manually, and you also have mocked data ready for testing eg, cypress.