Q:

How do two hosts connected to the same WI-FI communicate?

I have the following setup of two RPIs connected to a wireless router flashed with DD-WRT. 
If I ping the RPIs from the router I get around 4ms of delay. If I ping one RPI from the other i get around 8ms of delay. To me, this makes sense: RPI1->access point, access point->RPI2 (4 + 4 = 8) 

However, if I add additional delay to br0, or eth1 (the wireless card) using netem: 
tc qdisc add dev eth1 root netem delay 100ms. 
I obtain the following scenario:

How is this possible? 

A:

When the two RPi's communicate with each other, the access point does have to forward packets from one to the other. This is internal functionality of the access point function and has nothing to do with the fact that it presents a network interface to the operating system for packets sent and received locally.
You imposed a delay on the local network interface, not the internals of the access point function.

