SSE (Server Sent Events) connecting issues

[GoldenGamerLP]

Environment

Using NodeJs 21.6.2 and deployed on Vercel. Using chrome Version 124.0.6367.61 (Offizieller Build)

Reproduction

 * import { createEventStream, sendEventStream } from "h3";
 *
 * eventHandler((event) => {
 *   const eventStream = createEventStream(event);
 *
 *   // Send a message every second
 *   const interval = setInterval(async () => {
 *     await eventStream.push("Hello world");
 *   }, 1000);
 *
 *   // cleanup the interval and close the stream when the connection is terminated
 *   eventStream.onClosed(async () => {
 *     console.log("closing SSE...");
 *     clearInterval(interval);
 *     await eventStream.close();
 *   });
 *
 *   return eventStream.send();
 * });
 * ```
 
 - This is the default example

### Describe the bug

In a nutshell: The time it takes the client to connect with the SSE is between 10 and  up to 30 seconds. I dont know whats going on, but its not usable right now. I hope someone knows whats keeping the time to connect so high.

Regards

Alex

### Additional context

The source code on which Iam working on, but has the same issues:

import { addSyncClient, removeSyncClient } from "~/server/utils/sync";

export default eventHandler(async (event) => {
const { boardId } = getRouterParams(event);
const { uniqueFingerprint } = getQuery(event);

if (!boardId || !uniqueFingerprint || typeof uniqueFingerprint !== "string") {
throw createError({
status: 400,
statusText: "Bad Request",
});
}

const eventStream = createEventStream(event);

// Add the client to the sync clients
addSyncClient(uniqueFingerprint, boardId.toString(), event.context.user ?? undefined, eventStream);

// Implement keep-alive mechanism
const keepAliveInterval = setInterval(() => {
eventStream.push("\n\nkeep-alive\n\n");
}, 10000); // Send a keep-alive comment every 10 seconds

eventStream.onClosed(async () => {
clearInterval(keepAliveInterval); // Clear the keep-alive interval
removeSyncClient(boardId.toString(), uniqueFingerprint);
await eventStream.close();
});

return eventStream.send();
});

### Logs

_No response_

[GoldenGamerLP]

New clue: I've figured that the "return #send()" at the end of the event handler is the problem. On vercel, this methods times out.

[GoldenGamerLP]

Found the problem: https://stackoverflow.com/questions/76204143/sse-works-locally-but-not-when-deployed-to-vercel

My question now is, is there a way to implement the vercel/streaming api into h3?