This build still has the defective AC88U 5GHz Wi-Fi driver. It works for a random amount of time that can range from hours to days and then finally explodes with packet loss and in turn a significant latency increase.
It's very noticeable when it happens. You'll see degradation in YouTube videos. Gaming will report a significant latency increase and you'll start to notice the "warping" caused by packet loss. Downloading files take a throughput hit. You can validate with a quick ping to the default gateway and discover that you have incredibly variable latency to the gateway. In my case it can scale well into the 400ms range.
Resetting the router or toggling the 5GHz radio off and back on fixes it. The problem started with the 382 branch and continues to persist into this latest 384 build.
I had over 15 days of uptime on my own RT-AC88U without any wireless issue. Did you try the usual recommendations (disable MU-MIMO, Implicit Beamforming and Airtime Fairness), and using a different channel?