I was having the same jerky response here, and it was happening even when I removed LSP from the mix and just used the xLights RGB cycle "Mixed Colors" test on my etherdongle-connected pixels. My E681-connected pixels were nice and smooth, but my etherdongle pixels were pretty jerky (about one update per 1/2 second).
I discovered that the issue was due to a bug in the etherdongle firmware, as referenced here: You are not allowed to view links.
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LoginAs soon as I added another channel in the 1-512 range to the RGB cycle Mixed Colors test, everything turned real smooth.
As a work-around, I'm planning on adding a bunch of ramp up/down stuff to one of my unused channels in the 1-512 range within LSP.
Just thought I'd share this, as perhaps the mysterious jerky response others were seeing as described in this thread was actually caused by the same thing.
Andy