Sorry.
I was just reporting symptoms as I found them.
I changed everything with my system just before I added the new DisplayLink driver. Thus I was just going over the applications that I use.
I'm far from certain that the initial poster was seeing a different problem from me.
We are all using different versions of the OS, and the sluggishness problem could be evincing itself differently in different OS environments and with different library versions.
I tested a number of applications, and they don't evince the same behavior relative to the data points of builtin display versus display hanging off the docking station.
Yet the problem seems to relate to the docking display and the display link driver.
Chrome and totem movie viewer show sluggishness when connected to the display that the DisplayLink driver is managing but behave normally on the builtin display.
These two applications also behave normally if their output windows straddle the builtin and docking-station connected display.
Chromium and Firefox don't generally evince the problem, but I sometimes see sluggishness in backing up on an editable line with whole range of applications.
I did not report that one because I don't know how to cause it, and I don't seem to be seeing it right now.
I suspect some issue in a shared library used by some applications. It appears that hardware acceleration associated with graphics output is in some cases not being managed properly, and this problem is evincing itself on using the DisplayLink driver to manage a docking station.