The High Efficiency WLANs (HEW) Task Group (TG) has been developing since 2014 the IEEE 802.11ax amendment, the 6 th generation of wireless local area networks (WLANs). One major novel feature of IEEE 802.11ax standard is the specification of uplink multiuser multiple-input multiple-output (UL MU-MIMO) techniques to cope with the increase of UL data/video traffic from small-form devices in ultra-dense networks. In this paper, we first show that the UL MU-MIMO physical layer performance can be dramatically impaired due to the asynchronous reception over the uplink channel. Next, we propose and evaluate the performance of a time advance mechanism designed for IEEE 802.11ax WLANs that mitigates adequately the performance loss due to asynchronous UL MU-MIMO channels.