In order to see associated wifi clients, even if they don’t have a DHCP Client or have no ip, you have to ask the AP for associated wifi devices:
# Universal (Tested with OpenWRT 14.07 and 15.05.X)
iwinfo wlan0/wl0/ath0 assoclist
# Proprietary Broadcom (wl)
wl -i wl0 assoclist
# Proprietary Atheros (madwifi)
wlanconfig ath0 list sta
iw dev wlan0 station dump
This way you will also see the connection speed. For me this is looking like this:
# iwinfo wlan0 assoclist
12:34:56:78:9A:BC -26 dBm / -95 dBm (SNR 69) 1930 ms ago
RX: 24.0 MBit/s, MCS 0, 20MHz 3359 Pkts.
TX: 130.0 MBit/s, MCS 14, 20MHz, short GI 1209 Pkts.