WiFi can be tricky, even frustrating to isolate performance issues.
The 5th Gen APX uses 802.11n with theoretical max speeds in excess of 400Mbps, but if you are connecting on the 2.4Ghz frequency, it may slow to 200Mbps or less.
Also, how do you know which access point you are connected to when running speed tests? If your device retains a connection to the AC unit, but your distance causes deteriorated signal strength, that might explain the lower speeds.
Two ways come to mind on isolating the speeds and potential bottlenecks. First, plug in a computer via 1Gbps Ethernet to the units and run speed tests. This will tell you if there are network impediments slowing the connections. If the speeds are similar, then start looking at WiFi.
For WiFi, set the SSID slightly different on the 5th Gen access point. Something like NETWORK (main router) and NETWORK_N. Now connect to the alternate network signal and run the tests. In fact, for test purposes, I would suggest making the 5Ghz SSID different also, so NETWORK and NETWORK_5 for the main router, NETWORK_N, NETWORK_N5 for the secondary.
Best if you "forget" the other networks when you do this, to ensure it doesn't hop over during testing. This ensures you are in fact testing using the WiFi signal on the unit you are trying to test.
There are WiFi utilities you can use to test signal strength. One simple one is the Apple Airport Utility on iPhone or iPad. Go into the device settings and look for the Airport Utility app, enable Wi-Fi Scanner. Then, open the app and click on the Scan link at the top right, it will scan all network signals in range and give you all kinds of stats.
You can tell which is 2.4 vs 5Ghz by the channel (1-11 for 2.4, higher numbers for 5Ghz). You will see stats like RSSI and channel. RSSI is a negative value, with -50 or less being the strongest, -90 or higher being basically useless. You may also find a number of unidentified networks, these are typically 2.4Ghz devices like wireless phones, wireless keyboards and mice, bluetooth devices, even microwave ovens. This is noise that interferes with 2.4Ghz network performance.
So, using the combo of these two, you can note the network, channel, RSSI for each connection, plot it with speed test results, and understand your network better. You may also note these signal strengths oscillate, higher or lower at any point in time, this is the nature of wireless signals.
2.4Ghz WiFi has longer range, 5Ghz has more bandwidth (wider channels). 2.4Ghz penetrates walls and floors better. 2.4 is more susceptible to interference, but in densely populated areas, both bands can get congested.
Speaking of congestion, if neighbors use the same channels and the signals are stronger near the second access point, slowness could be from interference from their network, even on 5Ghz. Why? Because WiFi devices, both clients and access points must analyze every single packet (even from microwave ovens) in the air, determine if it is authorized, intended for that host or one of its clients, then processed or dropped. It is sloppy, and can delay real data transmissions.
Channels are a way to avoid the interference. Think of AM radio in your car at night out in the middle of nowhere. You often get sound from two stations and it is pretty annoying. That is because the radio signal travels farther at night, so you are more likely to be in range of two stations broadcasting on the same frequency (channel), and the radio is not designed to filter the signals. WiFi can filter the signals, but it does take time away from good signals, resulting in delays, re-transmits, etc.
All this to say, you may be able to improve speeds on the second access point by setting channels to avoid overlap with other strong signals within range. When using the scanner, test near each access point, and in areas where you typically use wireless devices. It can get tedious, but can help...until your neighbors do the same and trounce on your network again.
Frustrating, sure. But, even at 200Mbps, you probably have more than enough network speed (bandwidth) for everyday activities, so maybe you just leave it alone and live with it. But, the above can help you understand why performance is less in certain parts of the home.
