I managed to install the firmware for Xiaomi from http://downloads.openwrt.org/snapshots/trunk/ramips/mt7628/openwrt-ramips-mt7628-miwifi-nano-squashfs-sysupgrade.bin
But I did a comparison between using the stock firmware and the snapshot above and the snapshot isn't as stable. Example, I was able to stream YouTube from 5 devices concurrently but with the snapshot, I could only do with 3 at the same time. I'm using auto channel.
The auto channel setting did always use the lowest possible channel...
Download a App as WifiAnalyzer (Android) and check on which channels are the least of all concurrent ap's.
Set your channel to the best rated channel and test again.
Thanks juppin. It's a lot better now. I changed to channel 2, the least used channel. During the test, sometimes one or two devices would stop working. I mean like I can't can't access any sites or stream YouTube. I had to reconnect the WiFi for them to work again.