Once upon a time, back around 2000, there was a natural media vector program called "Expression", by a studio named Creature House. It did a lot of heavy, complex rendering, yet retained pretty speedy performance on the machines of the time. But Microsoft bought Creature House back around 2003 and ended support for it; "Expression" became a Windows-only tool with greatly limited functionality.

One of the ways it kept its performance snappy was by having an additional state for layers. Like Illustrator, they could be visible or invisible, and they could be locked or unlocked - but they could also be frozen.

Frozen was sort of a super-lock. A frozen layer was locked and uneditable. But freezing a layer was also an explicit hint to Expression's renderer: I am not planning to do anything to this for quite a while; please render it to a bitmap and cache it for speedy performance. You'd start to see pixels when you'd zoom in far enough but that was fine. Saving it as a jpeg/png/tiff/whatever would ignore the frozen image and render it at full resolution.

This was a great feature. I do some pretty complicated stuff now and I constantly miss it. All I can do now is turn a layer off or set it to render as outlines and that's nowhere near as useful as having a low-res proxy that I've explicitly asked Illustrator to use instead of rendering it again and again.

There were a lot of other great ideas in Expression. If a few artists on the Illustrator team got Expression 3.3 running in a virtual machine and played with it for a couple of weeks they'd probably find a lot of ways to make Illustrator a more capable program for artists.

(I would swear I asked for this before? I can't find it in the system. Maybe I asked for it in the old feature/bug form, before Uservoice. I've been using this thing as my main art medium too **** long.)