I recently bought ASUS RT-AC65P so I would able to use my 300mbps connection fully, as my older router was able to deliver only around 120mbps. On stock firmware I was seeing 300mpbs download and 400mbps upload on my laptop, but after flashing openwrt (OpenWrt SNAPSHOT, r13600-9a477b833a) the speeds dropped to around 120/200 on it.
I did nothing special in regards to configuration apart from trying to use SQM package but using it only made things worse (download dropped to around 80-90). I also think mu-mimo is not working as I loose wifi on my phone in my furthest room on openwrt whereas on stock firmware i got stable connection with around 250mbps (on 5Ghz).
Do you have any tips on making openwrt work at least so good as stock firmware? Do you know if mu-mimo is supported by openwrt on this router?
Which version do u have? spanish or british? There are numerous reasons, check for redundant processes. If load causes problems its not due to the processor its due to firmware. Reinstall, it might fix it. MU-MIMO is supported.
Most importantly check the clock speed of your CPU! If its clocked down return it to 880mhz
root@OpenWrt:~# dmesg | grep MHz
[ 0.000000] CPU Clock: 880MHz
In my case is 880MHz and I have very poor wifi download speed. Upload on 5Ghz is about 200Mbps (2/3 of ISP limit), but download between 20-60Mbps (ISP limit 1000Mbps, reachable via cable connection). Distance from router is lower than 50cm. I have almost last snapshot of oWRT OpenWrt SNAPSHOT r15666-8019c54d8a with luci manually installed. CPU idling during speedtest.
But range of WiFi is great, so it looks like software problem.