ASUS RT-58U slow WAN speed

I have a problem with my Asus RT-AC58U. Using original firmware all devices connected via cable or 5GHz Wifi are able to get full speed that i ISP is serving me (300/40Mbit. Unfortunately original firmware have lot of stability problems with 5GHz Wifi.

So, i've decided to try OpenWRT. I'm new user, but i was able to flash and configure my device using newest OpenWRT 19.07. After flashing i've only configured 2.4GHz and 5Ghz radio, security and some port forwarding, everything is working as expected, even 5GHz Wifi is rock stable. But unfortunately none of my devices are not able to get (300/40Mbit speed), they can only achieve (120/40Mbit).

I've found information that this could be related to cpu governor, i've installed irqbalance package, unfortunately it doesn't help, after that i've enabled "Software flow offloading" option from Firewall, but this doesn't help either.

Only thing that did improve something is changing
/sys/devices/system/cpu/cpufreq/ondemand/up_threshold to 35
/sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor to 10

Now devices are able to get around (200/40Mbit) but still this is not full speed. May I please have some help?

It would help if you perform the speed test on a wired connection, this is necessary in order to see if the bottleneck is due to WiFi eating too much CPU or due to routing eating too much CPU.

If it is WiFi, then you can try different variants of the ath10k driver and firmware (ct vs non-ct vs ct-htt).

I forgot to mention that I also did test using wired connection and speed were exactly the same. I've also tried benchmark connection between two PCs connected to router and speed was around 1Gb/s.