Primoz is right, if that AP is more affected by interference in the client band than on the other two. The System Info file (click first icon to the right of AP under Monitor/
Access Points page), is a text file you can read with Notepad++. Athstats are the
radio details (radio0 = 2.4, radio1 = 5G), and the Histogram shows a 2 minute
rolling RF sample. This data appears at the bottom of the files, just before AP
Secondly, since most modern clients can use all 11 of the 2.4 channels, you may
wish to employ the ChannelFly algorithm to manage channel assignment, (from
the Configure/Services, Self-Healing section). Having 3+ APs is helpful, and
ChannelFly also looks at non-802.11 interference, to help select the best of all
11 available 2.4G channels. You should see optimal throughput after Channelfly
has had a little time for your APs to build their internal channel/interference tables.
Lastly, compare Speedflex from the ZD to APs/clients, as well as your othe client
speed tests. You may find good local network times, but that your Internet
connection is the slow point.