@Kevin_Z, thanks for your response. I've actually been working with TP-Link support since I posted this. In short, yes I've ensured the routers are using different 5GHz channels and are both running the latest firmware to the best of my knowledge. The ArcherC1900 does not have an AP mode configuration but they are connected using your recommended setup (https://www.tp-link.com/en/support/faq/417/). As mentioned above, having the new unit in either router or AP mode makes no difference to the 5GHz performance. I've also run the new unit in standalone router mode without the old one connected with no change.

For the longer answer, I previously conducted troubleshooting using the FAQ (https://www.tp-link.com/en/support/faq/2170/) and determined the issue to be case 2 and supplied the relevant details. The wireless clients connect with a strong signal level (e.g. for a 802.11n client I connect with the following - Channel 149 (5GHz, 40 MHz), RSSI: -68dBm, Noise: -95dBm, Tx Rate: 243 Mbps, MCS Index: 20, NSS: 0). The poor performance is equally apparent in a variety of other wireless clients (Windows, macOS, iOS, Android).

What is strange to me is that while connected I can get small data transfers through like basic pings, most DNS lookups and even text messages on WhatsApp desktop on the router but once I try data transfers of substance (WhatsApp media messages, iMessages, e-mail, web pages, LAN file transfers) it fails. I sometimes wonder if it's an invisible traffic throttle in the router software or if it's a hardware issue but I have no way of doing any kind of debugs or advanced diagnosis to find out. I have done factory resets on the unit but that's about all I can check. Hopefully TP-Link support will shed some light on it when they get back to me.