6LowPAN was introduced by the IETF as a standard protocol to interconnect tiny and constrained devices across IPv6 clouds. 6LowPAN supports a QoS feature based on two priority bits. So far, little interest has been granted and this QoS feature and there are no implementations of such feature in real networks. In this paper,we evaluate the capacity to provide QoS of these priority bits in various scenarios. We show that under very heavy or very low network load, these bits have a limited effect on the delay.