Here is my experience:
The game reads my latency at around 50-70ms normally.
I am usually able to play the game fine. On occasion, the reading will jump to 200-600ms, 800-900ms, or 1500+ms. These jumps are temporary in length of duration and sporadic in terms of individual events. If I were to rate how often this happens, using in-game terminology: 200-600ms is rare, 800-900ms is legendary, and 1500+ is unique.
Now that I am conscious of this happening with this game only, the last time I was experiencing constant lagging with high latency readings that kept on happening, I teleported to town, then teleported back to the area and the lag and latency were gone. Other times I have logged out and logged back in and it is gone.
Maybe this is all coincidence. Maybe there is a problem with phases and instances. I turned my cross-network play setting to disabled and my game ran about 10ms lower on average and was always stable. I did that for a few days and then turned it back on and it didn’t take long before I noticed problems with lagging and latency again. Conjecture? Maybe. But, there sure are many reports of problems and there are also reports of Diablo 4 crashing people’s home networks. So, this game is doing something weird that other games are not.
If one is truly interested or concerned about figuring out what is happening; there is a dedicated thread and forum user (DTMace) conducting tests and gathering data: