You can build complex assemblies consisting of many components, which can be parts or other assemblies, called sub-assemblies.
All large scale design uses complex assemblies. You may need to simplify complex assemblies to improve performance, including creating simplified configurations of the assemblies.
Large Assembly Mode offers system options that improve assembly performance. You can enable it at any time, or you can set Large Assembly Mode to turn on when the assembly reaches a specified limit. You can customize it and combine it with other simplification methods, such as toggling the visibility or changing the suppression state.
Quick View, available with an open file provides a lightweight environment just for viewing your design. Use the "Selective Open" option to open a portion of the design to work on it after your initial visual review.
For more information about these methods, see SolidWorks Help > Assemblies.