D6000 dock lag on Ubuntu 18.04

Short answer: If your system has Thunderbolt 3 (it might be optional rather than standard on that model), you might have better luck with the TB16.

Long answer: I can't speak to Linux issues specifically, but the D6000 uses DisplayLink technology for video output rather than the GPU's native display outputs.  DisplayLink works by using the CPU and GPU to compress data intended for the displays, then transmit it as standard USB data traffic to the DisplayLink chip in the dock, which then decompresses it and sends it to the actual displays.  This means you can end up with a variety of undesirable side effects, such as compression artifact "blocks" and laggy/juddery motion, in certain circumstances.  Those circumstances include the CPU and/or GPU being heavily utilized for other purposes, the USB data bus being heavily utilized for other purposes, and/or changes to significant portions of the display area, e.g. when watching full screen video or gaming.  The potential for issues like this increases with the total resolution you are trying to drive from the host system.

As a result of these potential drawbacks, the D6000 typically is only really intended for a few use cases:
- Businesses that use "hotel desk" arrangements and want to deploy a single dock that will work with both systems that use the legacy underside E-Dock connector (by connecting the D6000 over USB-A) and newer systems that use USB-C/Thunderbolt.  This is the only Dell dock that fits that description.
- People who want to run more than dual 1080p on systems that only have regular USB-C rather than Thunderbolt 3.
- People who want to run more than dual 4K even if they do have Thunderbolt 3, since TB3 maxes out at dual 4K (currently).
- People who have systems that don't have a proper dock connector at all, since they can connect to the D6000 over USB-A.

For all other cases, the WD15 or TB16 docks are typically a better choice since they both tap into the GPU's own outputs and therefore would not have these drawbacks.  They also don't require DisplayLink software installation.

This is a great answer and directly to the point. I've found the D6000 pairs best when used with Chrome OS or Android as you don't have nearly as many background processes consuming CPU as you would on Windows/Linux/macOS. The higher powered Chromebooks like the Google Pixel LS (2015) or the Pixelbook can really take advantage of the built in DisplayLink support since R57 (which has been quite a while ago since they are on R71-ish as of 10/25/18).

If you have Thunderbolt 3 available the TB16 is far more reliable and better for multiple external displays at 1080p or 4k. The D6000's HDMI port will work without a driver if your device supports USB-C DisplayPort Alternate Mode, but with the lower bandwidth of USB-C vs Thunderbolt 3 you can't run 4K beyond 30Hz if I remember right.