Had the same problem with Mi Max 2. Now I got a router with 5ghz WiFi, no problems with Mi 9 and mi Max 2. If possible, use 5ghz WiFi, that's the only solution I know since 2,4ghz and Bluetooth show interferences/use the same internal antenna
I change the wi-fi router today... unfortunately with another 2.4 frequency. The same problem occurs....when Bluetooth is in use the wi-fi drops down to 4-5Mbps download rate . When I switch the Bluetooth off the wi-fi rate go to 100Mbps.
I am use mi band over an year and just in the last month I see this WiFi trouble so I'm thinking is some modification mades in the last versions. Anyway..
I will buy a 5mhz router but the question remains: is a general problem or the late ROM's is the main reason for this thing happened?
I made more research and with another phone (redmi note 2) the difference between WiFi with Bluetooth active is not present. So this is an argument for a rom bug and explain why I don't have this problem in the past.
So, with actual xiaomi.eu when Bluetooth is active the WiFi goes very slow(4-5Mbps). When Bluetooth is off I have 80Mbps.
With redmi note 2 with an old miui 10 on it I have 40Mbps with Bluetooth connected or not...with mi band2 connected or not.
In conclusion the interference between WiFi and Bluetooth is much more severe with this actually rom 9.5.16