How to send headers from Cloudflare worker before response body is ready?

The boiler-plate code for a new worker script looks like this (abbreviated)

async function handleRequest(request) {
const response = await fetch(request)
return response

As far as I understand, nothing is sent to the client until the sub-request completes. I would like to send some headers to the client before I make the subrequest.

(The idea is to send Link rel=preload headers, so that the browser can start loading css, javascript and fonts, while the worker is still making sub-requests and generating html)

Could someone give me some sample snippet to get started?

Thanks a lot