Just got off a support chat with "Jeffrey" where I reported the slow LTE speeds.
Here are some of his responses to my questions:
I'm getting slow LTE speeds on the iPhone XS.
Our engineers are aware of this issue. They are working hard at getting this fixed. They are asking you to make sure you keep your phone updated as the fix will come in an iOS update very soon.
What if it's a hardware issue? Some people are reporting no problems with WiFi or LTE speeds. Others like me are reporting many problems. So it may be a defective unit.
It is not hardware. If it was hardware I would be replacing your phone right now. It is an issue with certain Cellular Carriers and the way it speaks to the phones. I cannot speak to which carriers. All I can say is the fix is coming very soon. I promise a fix is coming very soon, keep your eyes open.
You always have 14 days from day of purchase to exchange or refund if you wish do so.
So the slow LTE may be a carrier software issue. My network is Verizon.
But that doesn't explain the people having WiFi problems. In my case I'm not having any WiFi speed issues.