Yeah that sounds unusual, the only thing that comes to mind is QoS (quality of service) it's a feature on some routers but is normally disabled, however on some it is enabled out of the box and what it does is give priority to different kinds network traffic.
QoS requires a lot of care in implementation so if some automatic QoS feature was turned on it may be doing more harm than good so I would turn it off. But this requires logging into the interface of your router and hunting around for the setting.