Slow WIFI speed - when second virtual WLAN is added

Hello,

Information: RBcAPL-2nD, with the newest stable firmware.
I have problem, because when i create second virtual WLAN (for guest) WLAN speed decrease to 2 Mbps (its not because of channel utilization etc.) - when second virtual interface is deleted everything back to normal - WLAN speed = 65 Mbps.
Only settings which I’ve change is VLAN tagging and passwords.

Is it bug or i have to change something in configuration? I do not report configuration because its almost default (after factory reset). Issue easy to reproduce i think.

Greets