1. What is the client devices signal like and TX rate?
2. Are you using 2.4 or 5Ghz?
3. Is there a lot of other wifi networks in the area?
4. Are the devices using the latest wifi drivers?
MX64 is working at it's maximum throughput, you will never be able to use the full 300Mbps using a MX64 as the max stateful throughput is 250Mbps yet it you have multiple clients then that speed is shared amongst them all so what you are seeing is probably accurate.
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