How many components are in your large assemblies?
What are your system Specs?
Upgrading your network cabling won't do anything for you unless you upgrade all of your switching equipment, server NICs, and data storage to take advantage of the bandwidth.
Edward, this response requires some multistep changes to ensure efficiency.
Step 1. Design for Large Large Assembly- Learn all the techniques to design with large assemblies in mind.
Step 2. Use EPDM. this will allow yout to work local and minimize network inefficencies.
This is where I would start, contact your VAR and see if they will help you evaluate your large assembly practices.
If you are using Professional or Premium version of SW I would seriously consider using the Workgroup PDM add-in. The immediate benefits are the same as EPDM without the hefty price-tag (it is already included with Pro and Premium).