O culpado pode ser a Certificação de economia de energia.
De Wikipedia :
The underlying concept of WMM PowerSave is that the station (STA)
triggers the release of buffered data from the access point (AP) by
sending an uplink data frame. Upon receipt of such a data (trigger)
frame the AP releases previously buffered data stored in each of
its queues.
O AP estaria, assim, atrasando os pacotes até "instruídos" a liberá-los.