Re: WD15 and Latitude 5280 - Slow internet speeds 8mbps
It's true that the WD15 does not use DisplayLink; it taps in the native GPU output wired to the USB-C port instead. That means that unlike the D6000 it can't be used on USB-A ("regular USB"), at least not if you want display output and system charging, but DisplayLink has its own share of drawbacks, but those aren't worth getting into in this post.
Yes, the WD15 has both 130W and 180W versions (not sure where you found a 210W adapter as you mentioned in your second post, especially as the part number you mentioned indicates 240W), but you can consult the WD15 FAQ to find out which systems require which type of power supply. You are typically not supposed to have AC adapters connected directly to the system while you also have the WD15 connected, however. Same goes for the D6000 when you're using it with systems that are connecting to it via USB-C and can operate at full performance with no more than 60W of power. I doubt that would be the cause of your slow wired LAN performance, but just fyi. You're also definitely not supposed to have a 65W AC adapter connected to the WD15 as you mentioned for "Laptop #5" in your second post.
Lastly, in your third post, you mention connecting WD15s to Precision 3510s using USB 3.0. Either you meant to say USB-C there, or you're connecting the WD15 in a completely unsupported manner. There are no official adapters that allow a USB-C peripheral to be connected to a USB 3.0 port, i.e. USB-A. Some specific peripherals come with such an adapter if they have been explicitly designed to allow operating that way, but that cannot be assumed, and unlike the D6000, the WD15 is not one such peripheral. The WD15 is meant to be connected only via USB-C.
With respect to your actual issue, your three posts are a bit hard to follow because it's not clear what relationship (if any) there is between the systems and docks mentioned across the three posts, and additionally in Post #2 you break things down by laptop, whereas in Post #3 you break things down by dock. But I'll read it over a bit more to see if I can find a common factor.