MT 5Ghz wi-fi and regulatory-domain / country=Russia - what is the reason for the low data transfer rate?

Question

WiFi

There was a situation here - I hit my head against the wall for a long time, trying to understand the reason for the ugly work of the waffle, despite the fact that there are 4 clients, and all in the same room as the router, in direct line of sight.
As a result, it was worth changing the country from Russia to United States - the data transfer speed increased 6 (!) Times and, most importantly, stabilized. The signal strength of the clients both was and remained at the same time, that is, it is definitely not a matter of limiting the transmitter power. The channel has remained as it was - 5180 emnip. How is this to be understood?

//MikroTik wAP ac LTE kit
//ROS 6.47.1