A useful tool that Ive used in the past with Hammer Editor was the cordon tool. It acted as a volume which everything outside of it isnt rendered, and is not processed in the compiling of the map. This allowed for only very specific areas having light built, rendered, and allowed the user to hide areas of the level that are not needed at the time. This tool helped with performance, as well as allowing you to not waste tons of time on building an entire levels lighting, just to see how a specific section will look.
That’s not what that’s for. Lightmass importance volume just focuses the quality of the lighting calculation, but it still builds lighting for areas outside of it.
Viper is correct. If I recall correctly, anything outside of the volume will only receive one bounce; rather than the N number you have it set to. Single bounces are pretty cheap, so I doubt there would be some landslide of difference between compilation times; regarding lightmass volumes vs the cordon tool(unless you’re doing something like squaring off a 30x30m area out of a 3000x3000m zone or something along those lines).