What sort of difference in performance did you see when disabling shadow casting?
Before materials/reflections, I’d wonder about overall polycount. If you look at these custom assets in wireframe view - do they seem reasonable? (not extremely dense)
I forget if it’s on by default or not - but if you go to Edit → Editor Preferences → Performance there is a checkbox for ‘Show Frame Rate and Memory’ - With this enabled, you might be able to troubleshoot in-editor instead of needing to launch a session every time.