Having the same issue on 200mbps broadband. Get full 200mbps on Ethernet but only around 30mbps on WiFi.
Tried these different setups.
Hub 3 (Router Mode) - No luck
Hub 3 (Router Mode) ----> UniFi AC Pro Wi-Fi Access Point - Still no luck, still limited Wireless speeds which is strange
Hub 3 (Modem Mode) ----> Router ----> UniFi AC Pro Wi-Fi Access Point - Still limited
Don't understand why this is happening but seems to be something wrong with the Hub 3 if even with a seperate router and access point connected, the wireless speeds are still limited to exactly the same as if you're connected directly to the Hub via Wi-Fi.
I too have noted the limitation in speeds even when using a different router with the hub in modem mode.
To test I have used an iPhone 6s+, iPad Air 2 and a Samsung Galaxy S7 when I have tested it. All of these devices are capable of achieving a 1300Mbps link speed to the router. I have also multiple times tested the wireless speeds on a SuperHub 2 modem which easily achieves the 100Mbps.
Prior to "upgrading" to the SuperHub 3, I used the same wireless router to connect my wireless devices and I was able to achieve the 100Mbps so it is really confusing and frustrating.
Maybe there is a 5g Hz speed setting in the router but I can't think where or why. I'd look at the hub gui for possible suspects or reset to defaults options, as multiple devices are being affected. But on a wired access point, the hub WiFi is bypassed, so it puzzling
I assume you have already tried changing the WiFi channel, in case it is due to interference from nearby