Thanks for posting!
Any USB-connected Ethernet adapter (not just our dock) has the potential to be slower than a built-in Ethernet port due to the different connection types. That said, if the connection is not stable there could be outside factors contributing to the behavior and/or performance level.
To that end, can you confirm you are using the USB-C cable that was included in the box with the dock to connect it to your system? I ask because in some cases 3rd-party cables can cause problems similar to what you describe…
You may have already tried this, but if you have a new Ethernet cable available, swapping it in will help rule out the current Ethernet cable as the source of the problem.