Sometimes a websocket message from Espruino WiFi is invalid UTF-8? Crashes the connection.

It also seems like up until the this crash happens, the network responses take longer and longer until the crash. I'm keeping a running average, f.e.

    let ws = new WebSocket("ws://192.168.43.247/my_websocket", "protocolOne");
    let lastTime = performance.now()
    let timesCount = 0
    let totalTime = 0
    let averageTime = 0
    ws.addEventListener('message', ({data}) => {
        timesCount++

        const thisTime = performance.now()
        const elapsed = thisTime - lastTime

        totalTime += elapsed
        averageTime = totalTime / timesCount

        lastTime = thisTime

        mpuData = parseEspruinoJson(data)
    });
    ws.addEventListener('open', () => {
        ws.send("Hello to Espruino!");
    })

    setInterval(() => console.log('avg time:', averageTime), 2000)

and at first the average is about 38 ms for the time between responses, and by the time it crashes the running average is around 50 ms. It increases steadily the whole time.