I think that probably depends on whatever is running in the background when your screensaver launches. When you developed it as a normal channel, you were giving it full priority as the only thing running on the system. When it runs as a screensaver, however, it's in a separate thread that doesn't get the same priority as the main thread. I've noticed that having an roAudioPlayer running in the background has a significant impact on the performance of an roImageCanvas. I also noted in another thread that the roOneLineDialog also eats up a lot of CPU when the ShowBusyAnimation is being displayed, so there are any number of things that could be running in the main thread that could hinder the performance of the screensaver thread.
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