Nighthawk X4 (R7500v2) AP Mode Performance

I picked up an X4 router to use as a second access point in my home, replacing a very old Buffalo WZR-600DHP router that has performed well for years.  My thinking was the newer technology would help extend the range a little and give me better performance overall.

I have Verizon FIOS and use the Verizon G1100 as the main router, DHCP server and access point.  However, it is tucked into a remote corner of my house, and therefore the wifi service was initially very poor in the other end, which is why I set up this system many years ago. I ran a physical ethernet cable to the far side of the house, and used a second router (the Buffalo) as an access point only.

When I installed the new X4, I set it up the same way as the old Buffalo.  Because my older router did not have an "Access Point mode", I always had the cable plugged into one of the LAN ports, not the WAN/internet port.  DHCP is turned off on the secondary AP router, so that both routers and alll devices are on the same subnet.  Both routers have static IP addresses assigned (outside of the range handled dynamically by the DHCP server) to avoid any conflicts.

The setup works, but performance is SLOW.  My Verizon service has 75 Mbps download (verified through a wired connection to a computer).  If I connect wirelessly to the X4, I'm luckly to get 15 or 20 on either the 2.4 Ghz or 5 GHz band.  If I use a wired connection to the X4, I get 75 Mbps or more, so it's not the fault of the cable or anything upstream. It's not a range or interference issue either -- this is when I'm sitting right beside the router!  I'm on the latest firmware (V1.0.3.48), and have tried various channels and other settings, to no avail.  I also tried using the actual "Access Point mode" and switching the incoming internet to the WAN port.  No difference.

Finally, I tried adding back the old Buffalo router, connected with a LAN-LAN connection out of the X4, broadcasting on a different SSID.  At close range, the 5 GHZ service was nearly 75 Mbps, fully two or three times the speed of the new X4!

With all the newer technology and features, WHY is the X4 getting outperformned by a nearly 10 year old router??