ReallyBusyForm.Show();
ReallyBusyForm.Focus();

var floatingWindow = new FloatingLoadingWindow();
Task.Run(() => floatingWindow.ShowDialog());
await Task.Run(() => ReallyBusyForm.LoadYourControls());
Invoke((MethodInvoker)(() =>
{
    while (Application.OpenForms["FloatingLoadingWindow"] == null)
    {
        Thread.Sleep(200);
    }
    floatingWindow.Close();
}));

floatingWindow.ShowDialog(); //this call obviously block the execution of what comes next
await Task.Run(() => ReallyBusyForm.LoadYourControls());