Oddly, I’ve noticed that there are some GMIC filters which run for long periods without timing out the preview. This initially led me to think that the preview timeout lengths were programmed into - or omitted from - each filter separately by the filter’s author. But David’s explanation indicates that is not the case.
But I definitely like David’s idea of putting a timeout ‘override’ option into GMIC’s Preferences.
Even though it’s true that the GMIC plugin will not always render its results exactly the way the preview shows them, it is nonetheless extremely useful and informative for the preview to, at least, give an idea of ‘what to expect’ when a parameter is changed. There have been many times when my ‘experimenting’ with a variety of settings has been frustrated by consecutive preview timeouts. In times like those, I give credit to those experienced users who give the common-sense advice to simply apply the plugin to the image to see the end result.