WiFi performance issues with netgear R7800

I have installed openwrt (trying both the official and hnyman's builds) on my r7800 and I am having wifi performance issues. Mostly, speeds are relatively slow (although I haven't done a proper test) and connections are unstable (dropping out entirely sometimes, but usually the device stays connected and simply can't communicate with any others on the network).
I have tried tweaking usual wireless options (band, transmit power) without much improvement. This seems to be more of a problem with the linux and android based devices on the network than with the windows ones, although I can't confirm that this is the case. The ISP-provided router did not have these issues so I am assuming interference etc are not the problem here.

I would appreciate any help!