Just changing from a to n to ac, without clear purpose, will not help much. It is not possible to find the good setting with just "trial and error".
Wifi works at different interface rates (ac delivering the highest rate for Mikrotik). Multistream can multiply this interface rate. All must be supported by both sides to be used.
What interface rate is used depends on the signal strength and quality (distance, walls, ...) and the noise level (other devices emitting on other channels)
In that interface rate a lot of throughput get lost by wifi overhead (50% etc), and only one device can talk at the same time. They wait for each other. Co-channel coexistence. Any slow transmitter (low interface rate) will take a large part of the available airtime. Even if your device is fast, it will be held up by any slow transmitter in the area.
To remediate slow wifi, one has to know what the major cause is. Any setup is different, and it may vary during the day.
Mikrotik has tools to find out what the working condition is. Check the interface rate (registration table) , and check the environment (scan, snooper, freq usage)
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