You could also try the "Force path tracing" option if time to first pixel is an issue, as this disables calculating the cache in IR only (leaving your final render settings untouched), the purpose being to reduce time to first pixel. If you use an Interactive Viewport (rather than the VFB), you can lower the quality there, it defaults to 50% which should be good, which means it renders the image at half size and upscales it (in a non-AI, everyday way) - may be good enough for doing things like placing vegetation, etc. For the VFB, you could try setting the resolution to half size, and in Corona Preferences raising Image upscaling factor to 2 - same thing, makes the image half size but upscales (non-AI, everyday) it in the VFB though in this case, remember to set the final resolution back before doing a final render unlike the viewport version.
This will depend somewhat on what is taking the time to process, if it's some pre-parsing like calculating displacement or geometry etc. these things may not help as much; about the only thought I have there is to group the scene in such a way that you can turn off elements that are not required when working on certain things.