Slow Wi-Fi Speed on AC88U Specifically

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New Around Here
I have an issue with the Wi-Fi speed on my AC88U running real scenario tests.

So for the purposes of the test I'm running 4 devices:

ASUS AC88U (runs around 60 degrees C during testing (with cooling) - 2.4Ghz & 5Ghz, around 6 WiFi devices reported on Merlin)
Macbook Pro 2016 (Wireless AC & Gigabit Adapter)
Windows 10 PC (intel 7940x & Wireless AC & AD, 10G Ethernet & Infiniband 40Gbps)
NAS (Infiniband 40Gbps & 2x 1G Ethernet (Load Balanced))

All devices are in the same room, no obstructions and limited interference.

So obviously over the Infiniband connection the NAS to PC reaches in the region of 450MB/s transfer speed which is expected as I've reached the limits of the RAID hard drives at that point.

Connecting both the MacBook & PC to the NAS over Ethernet through the AC88U yields transfers of about 115MB/s each: Expected, its only Gigabit Ethernet and theres SMB overhead etc.

Connecting the MacBook Directly to the PC over Wireless AC (Ad-Hoc) is similar with 100MB/s: still decent, it is Wireless after all though still very low ping.

Though the problems start when connecting either the MacBook or the PC to the NAS using Wireless AC through the AC88U.

The ping between the laptop and both the router and the NAS is within the region of 3-5ms and similarly this is reflected with file transfer around 30MB/s? It's worse on the PC at around 25MB/s.

The question is if the laptop to PC connected can get around 100MB/s over Wireless AC transferring the same large files, running the same Crystal Disk and Black Magic tests then why if I try the same thing through the router does the speed drop by a third. Is there any way I can boost the routers performance using Merlin to handle this WiFi -> Ethernet traffic better?

The TX speed on the MacBook is 1300Mbps (says OS X). In perfect conditions this should be faster than Ethernet and perfect conditions are basically there as evidenced by the PC -> MacBook direct speeds (when factoring in SMB etc)

There is basically no 5Ghz traffic in my area which is a bonus and the RSSI is around -25dBm so my only idea is that there is some problem between the AP and the Switch within the router. A fully ethernet connection delivers expected results though routing Wi-Fi into Ethernet within the router causes bandwidth to drop to a third. I'd say it was the CPU in the router not handling it but it's never more than 10% on both cores?

It is possible that the transmission power on the router is clipping (though I did a complete system reflash the last time I played with the TX power), though I don't know how to test it? Would disabling the separate SSID 2.4GHz help?

TLDR; Wifi between Motherboard and Laptop (direct AC connection) is decently fast. Using the router in between nets a third (at best) of the transfer speed.

Please advise.

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