@paulvw3,

When the Archer routers are placed into AP mode, the network and addresses are still managed by the router at the head of your network. 

There will already be a falloff in the speed of the connection due to it being wireless. It really is not uncommon to receive only 30-60% of what your network is capable of due to interference, and wireless overhead.

With the device wired as an AP, it is also possible that the main router is managing the connection and total bandwidth of the ports, saving additional bandwidth for the connections that will occur on the AP, and also on devices connected directly to the main router. Make sure that there are no QoS settings on your other router that could be affecting the connection.