High number of shadows and shadow effects (such as shadow quality).

Having too many lights with their shadows turned on, as well as having too many shadow effects enabled, may slow down performance. 

Recommended number of shadows & effects for fastest performance

Desktop 3D:

• 2 shadows, 2 effects or less for fastest performance; 
• 3-4 shadows and 3-4 effects maximum

Mobile 3D:

• 0 shadows for fastest performance on all devices 
• 1-2 shadows and 1-2 effects maximum 

AR:

• 0; shadows are not rendered in AR.
  Your scene may still contain shadows - they will simply be ignored in the AR viewer. 

VR:

• 2 shadows, 2 effects for fastest performance; 
• 3-4 shadows and 3-4 effects maximum

How to turn off shadows + shadow effects

To disable shadows, select lights and turn off the "Cast Shadow" setting in the lighting properties panel. 

To remove effects, select RT Render Settings in the camera properties panel, and turn off the "Shadow" setting. You can also decrease the shadow resolution if applicable. 

The overall performance of your BRIO scenes depends on setting constraints for all factors in your scene. View the full Performance Guide Overview for more details.