I am trying to set up SSE between a NodeJS Server and a TypeScript Frontend. My Server-code looks as following:

let sseInterval: { [index: string]: NodeJS.Timer } = {};
this.app.get('/events/:id', (req, res) => {
    const headers = {
        'Content-Type': 'text/event-stream',
        'Connection': 'keep-alive',
        'Cache-Control': 'no-cache'
    };
    res.writeHead(200, headers);

    const instanceId = req.params.id;
    const instance = this.findInstance(instanceId);

    if (!instance) {
        global.logger.debug('sse closed', {metadata: {for: instanceId}})
        clearInterval(sseInterval[instanceId]);
        res.end();
        return;
    }

    global.logger.debug('sse connected', {metadata: {for: instanceId, instance: instance.config()}})
    res.write(`data: ${JSON.stringify(instance.all())}nn`);

    sseInterval[instanceId] = setInterval(() => {
        global.logger.debug('sse interval', {metadata: {for: instanceId}})
        res.write(`data: ${JSON.stringify(instance.all())}nn`);
    }, 1000);


    req.on('close', () => {
        clearInterval(sseInterval[instanceId]);
        res.end();
    });
});

For the frontend, i am using the following code:

const datapolling = () => {
    let eventSource: EventSource = new EventSource(window.server.url + '/events/' + window.server.key);

    eventSource.onopen = (event) => {
        console.log('connected', event);
    }

    eventSource.onmessage = (event) => {
        console.log('onmessage', new Date(), event);
        const json = JSON.parse(event.data);

        updateGUI(json);

    };

    eventSource.onerror = (event) => {
        console.error('error', event);

        setTimeout(datapolling, 500);
    };

    window.addEventListener('beforeunload', () => {
        eventSource.close();
    });
}

console.log('polling', new Date());
datapolling();

The problem is, that the frontend only receives the evenets every 3000ms but 3-4 at once. If i set the backend intervall to 500ms, i get the frontend events every 1500ms, again 3-4 at once.

Is there anything obvious i am doing wrong?