Guest WiFi network much slower than normal network

Speedtests from a device connected to the guest network 5GHz WiFi are much slower than when the same device is connected to the normal 5GHz WiFi.

I reproducibly measured around 7ms 18/10 Mbps up/down on the guest network and 1ms 320/250 Mbps on the normal network. Same location, same device, same speedtest app, same test server.

Similar results from several devices (Windows laptop, iPhone 6S). No change after a reboot. QoS is switched off. Firmware is 1.0.6.28_1.1.83.

What could be the reason? I don't think I had the same problems a few months ago, but I didn't change any settings since then, except upgrading the firmware when a new release popped up.

Thanks for your advice.