OnePlus support had my phone for 3 weeks to fix my 5G wifi speed problem. They report that they replaced the motherboard under warranty and the phone has been shipped back to me. I should have it in a couple days and will certainly test to confirm that 5G wifi speed is similar to other devices connected to my network.
I've defined separate SSIDs for 2.4g and 5g wifi in my gateway, so I can control which SSID I connect to. 2.4g was always consistent with other devices connected at 2.4g. The problem with my phone was related only to 5g.
I received my "fixed" phone about a week ago now and I've been monitoring its wifi performance. Bottom line: 5G has been generally slow, much slower than my laptop or my Moto Ace 5G, but the speed is highly variable and quite fast sometimes. I just ran Speedtest on my laptop and OnePlus. Both were connected to the same 5G SSID. The laptop performance was 304 mbps down, 172 mbps up. The OnePlus performance was 48 mbps down, 205 mbps up.
I switched both the phone and the laptop to my 2.4G SSID. Laptop performance was 43 mbps down and 15 up. Phone performance was 116 mbps down and 88 mbps up.
After the 2.4G test, I switched the phone and the laptop back to the 5G SSID and immediately retested wifi speed. I observed performance of 339 mbps down and 373 mbps up on the phone. The laptop speed was 248 mbps down and 148 mbps up.
I know that my gateway broadcasts 5G on two different channels for the same SSID and I have no control over what channels it picks. Maybe the phone picks a congested channel sometimes and the slow observed speeds are due to that, whereas the laptop (and the Moto Ace) always seem to pick the less congested channel?
The other thing I read about was "randomized MAC address" versus "device MAC address" and someone, somewhere (I don't remember where) suggested that using device MAC address resulted in improved speed. I just made that switch for the 5G SSID. Speed is very good now, and I'll monitor it to see if it slows over time.
Sorry for the long message, but I'm thinking that the "fix" really did nothing to improve 5G wifi performance and there is something not quite right about the OnePlus's handling of 5G wifi.