This paper presents an in-depth analysis on the effect of the coexistence in the 2.4 GHz band between IEEE 802.15.4 and IEEE 802.11 on the medium access control. We focus on the slotted CSMA/CA medium access algorithm that is used by IEEE 802.15.4 and analyse the delay that frames undergo when suffering from interference from a WiFi activity. We measure the overhead caused by the additional delay spent in the MAC sublayer frame queue before accessing the medium due to the presence of WiFi interference. We experimented different scenarios with overlapping channels and non-overlapping channels. We show that the two wireless protocols can coexist if we take into considerations the relative positions of the nodes to avoid very high interference and if we avoid monopolizing the channel with a very high rate WiFi traffic even under overlapping channels.