In general you should look first at the slower task work areas. The concept behind Non-synchronous assembly is that it allows for buffering in slower task work areas, while maintaining continuous flow on the main production line.
What tasks can be re-tooled and moved into synchronous assembly manufacturing?
Can you address or initiate component or assembly level design changes that can have positive net improvements in manufacturing costing?
Training?
I assume you have already completed task time studies?