QNetworkAccessManager extremely slow on first call of get() with HTTP2

I have come across instances when calling QNetworkAccessManager::get() for the first time, responses arrive extrememly slowly. Take the below snippet:

MyClass::MyClass() {
    manager_ = new QNetworkAccessManager(this);
    manager_->setAutoDeleteReplies(true);
}

void MyClass::make_call() {
    QNetworkRequest request;
    request.setAttribute(QNetworkRequest::HTTP2AllowedAttribute, true);
    request.setUrl(QUrl("https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png"));

    qDebug() << "Starting request";
    QNetworkReply *reply = manager_->get(request);

    connect(reply, &QNetworkReply::finished,
            this, &MyClass::reply_finished);
}

void MyClass::reply_finished() {
    qDebug() << "Request finished!";
    make_call();
}

I just measured (very approximately with my phone stopwatch!) the time between printing "Starting request" and "Request finished!" to be just over 1 minute.  After this extremely slow response time, subsequent get() calls take well under 1 second.

This appears to have related symptoms to to QTBUG-33315 that was closed due to being a duplicate of QTBUG-10106, but I cannot see how this is related - hence the new issue.

Mitigations Found so far:

• Running the code after rebooting my development machine seems to make the code run fast again.  I am unsure at what point the call slows down/unable to reproduce this condition.  As far as I can tell it happens after my machine has been up for about 10 minutes (I can repeatedly run/clean build straight after boot but at some point it stops working).
• Removing the request.setAttribute(...); in the above snippet causes the get() call to run in well under 1 second first time as one would expect.  This is regardless of whether the issue is present with HTTP 2 enabled.