Dear @Matasar,

I tried to disable QoS \ WMM and it worked! No more lags in the game, but instead I've got huge speed drop, down to 20 mbits\sec. 

If you disable WMM, then wireless devices connected to the EAP can only get a maximum wireless rate of 54Mbps (802.11g/a mode). So the speed drops down to 20Mbps after you disable WMM on the EAP, it's normal. Similar queries about WMM can be found in the posts below.

Post #16, https://community.tp-link.com/en/business/forum/topic/101632?replyId=377390

Post #6, https://community.tp-link.com/en/business/forum/topic/150177?replyId=393008

So, very sad situation here: with WMM on I can't play my favorite MMO, but without I have slow WiFi.

You may enable WMM and enable No Acknowledgment(it's under WMM) to have a check.

Dear @Matasar,

Once I enabled the No Acknowledgment feature I faced with another problem: my devices are connected to the AP, but my network became unreachable.

I see, then leave the No Acknowledgment feature disabled, there is no need to enable it in your WiFi network.

Except for the World of Warcraft game, do you have the lag issue on other games or applications when the WMM is enabled on the EAP? Or do you have the lag issue on other clients?

As you may know, WMM is a kind of QoS(quality of service) setting, which can make voice/video/audio get prioritized over regular data traffic, so it is ideal for applications such as voice over IP(VoIP) and streaming media. What's more, the wireless clients need to support WMM as well to make the WMM function correctly.

Regard with the World of Warcraft Lag/Latency problem, you may also check the settings on your laptop, I found there are many discussion about the latency problem of the game on the Internet, you may search it from the Internet to check more details.

https://www.pcerror-fix.com/fix-world-of-warcraft-latency-lag-disconnects