Upgrade to SW2015. Use large assembly mode in combination with speedpak, Make sub-assemblies and keep number of mates to less. Delete all unwanted display states and configurations.
Make parts fixed and supresss mate, Open assembly in lightweight mode.
1) Use large assembly mode,
2) check auto load lightweight
3) suppress tool box components
When you have the assembly open, run the Assembly Visualization tool and make note of which components are taking excessive time in the SW-Rebuild Time column. Then open those components to see if problems exist.
If the component is a sub-assembly, use the Assembly Visualization tool again and repeat the process.
If the component is a part, use the Feature Statistics tool to see which feature is taking excessive time.
In both cases, resolve all issues and, if possible, create simplified configurations and reduce the level of detail required.