When doing very complicated artwork, sometimes Illustrator's live preview starts to become impossibly slow. Distorted dense fill patterns, lots of art brushes full of translucent shapes, complicated high-res bitmap effects… I can do some stunning things with these but I’m reluctant to use them, because I need to have fast response time in my drawing tool.
Currently, layers can be locked, or unlocked. I would like to have a third mode: 'frozen', where the layer is locked, and a bitmap of the entire layer is cached. This bitmap is then used for rendering the preview, until such time as the layer is unfrozen.
I imagine this being invoked by shift- or control-clicking on the layer's visibility eye in the layers palette; the eye would be replaced by a snowflake or some other indication of it being 'frozen'.
Frozen bitmaps are not saved along with the rest of the file; they’re regenerated when you load it.
(This idea shamelessly swiped from the late lamented Expression, which let me play around with natural media effects at workable speed back around 2005.)
Margaret Trauth commented
Oh, and since I am thinking about this again: the cached bitmap for a 'frozen' layer is NOT used when rendering a bitmap via anything under file>export. It is ONLY for speeding up the preview.