Agreed.UPnP should be automatic for any modern router. You should not need port forwarding. I've never needed it.
If however, it made a difference (the improvement might have just been random luck I guess), it might indicate that the router isn't 100% healthy/happy.
Is your router from a decent brand @Einar Donarsson, or an ISP-supplied model?
Sometimes problems of this general flavour go away for a while when you reboot the router. If that makes a difference then acquiring a better router might be worth considering.