On the osx upload side, there are no drops, essentially. My guess is all the latency is coming from a fixed length queue there, and when they run out, they push back on the stack.
Two ways to test that - I can't find a way to lock the mcs rate in osx with a few searches of the web, but narrowing the channel width at the AP to HT20 might be revealing. Going from VHT80 to HT20 would probably quadruple the latency observed from the osx upload if that theory is correct.
Polling the netstat -qq -I the_interface and dumping that somewhere during that upload test might be revealing also.
for i in `seq 1 60`
do
netstat -qq -I the_theinterface
sleep 1
done > fq_codel_osx.log