How to improve link performance in multiple unicast

In an indoor environment, I have more than 10 clients are connecting to an AP, pinging 50000 byte of packet size.

Then I add another 10 clients, pinging 50000 byte of packet size. After running few hours, start to see 3-4 devices encountered tx-ccq low % (eg. 1 - 5%).

I tested the AP with LANforge tool, it can support up to 50-60 concurrent users.

However, this is not tally with what I tested in real environment.

Not sure what is the root cause of this dropped tx-ccq issue.

Any idea to improve the multiple unicast performance in this AP? Any advise is appreciated.