According to Carlo, for some of the demanding screen savers, Apple requires full hardware acceleration. DisplayLink adapters are just adapters, and don't have GPU's (graphics processing units). Therefore, DisplayLink adapters can't do their own hardware acceleration; they just use the CPU, which isn't very good for graphics-intense things. You've probably noticed that watching a movie on your DisplayLink-powered monitor is more sluggish than on your other (GPU-powered) monitor.
It's probably possible to have the DisplayLink adapters use the GPU in your computer instead of the CPU, and then implement all of the special functions needed for OS X to use "full hardware acceleration mode," but that would be really hard to do! There's a lot of special functions, and a lot of different underlying GPU's that you would have to depend on!
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