The aim of this paper is to evaluate the real-time performance of our new FF-MAC 'Fast-Forward MAC' protocol through a set of real world experiments. This protocol is proposed as an enhancement of the IEEE 802.15.4 standard for time critical applications. Hence, it is compared with the GTS 'Guaranteed Time Slot' mechanism of this standard. The obtained experimental results by using real world wireless sensor network nodes are proposed as a complement to the simulation results, previously measured in the Network Simulator 2 'NS-2', in order to prove these improvements from a practical point of view. We introduced a new mechanism to the proposed protocol in order to meet the memory constraint. The goal is to protect the time critical data from being dropped when the coordinator queue is full. The obtained results confirm the viability of our protocol and its performances even in a realistic testing environment, which make it applicable in real world applications.