that would be Forest Pack Pro making your scene impossible. Espeacially in IR.
Forest Packs reloading when selecting a different camera
Using point cloud display is junk, use proxy-plant display for trees
Have all Forest Pack Pro geometry turned to Proxies (make sure to avoid adding crp_ as prefix or ur Forest Packs will get broken)
Dont use "edge mode" unless it's really necessary, it make (re-)computing the scene for rendering ALOT heavier
The non-responsiveness can prob be part of your viewport being to heavy from those Forest Packs, when doing a render in IR with Forest Packs enabled, you will see the Forest Packs vanish from the viewport. You can make them appear by selecting them from an explorer window (layer explorer, etc...) whilst doing IR.
Again: I'm looking at Forest Pack Pro for this... Not Corona.
Grts