The Realtek people are placing the blame on the XHCI USB3 controller used by the dock.
Except the issue they know about supposedly only applies to the TB16 dock, not the WD15. But I beg to differ..
Today I did 100's of GByte copies between various SSDs over USB/UASP. When copying between two USB3 ports on the dock I get a ton of timeouts and various other errors hidden away in systemd's logs. The kernel retries everything just fine, but it really REALLY slows things down.
Copying from a dock'd drive to one plugged directly into a Type-A on the notebook has no such issues. And single drive copy (from internal SSD) is also fine.
I also noticed that when I invoke a strong stress test on the RealTek USB-Ethernet adapter in the dock, I can produce random-ish PCIe AER retry reports pertaining to the XHCI controller for the dock. My suspicion is that backflow control in the PCIe or XHCI of the dock isn't configured correctly by the dock firmware. Either that or the hardware is simply buggy.
None of this kills my daily routine or costs me data (yet), so it's more of a nuisance than anything else.