A company or an organization may have a number of business processes depending on its size, represented in a particular business process modeling language such as BPMN. They may have inter-operating relationships, i.e. one process may depend on other processes by exchanging messages in order to achieve its objective(s). One of the process improvements is to optimize its processing time, e.g. by parallelizing some tasks within a particular process. Optimizing processes individually without considering any inter-operating relationship between them will potentially produce a process which is not consistent with the original one w.r.t. the pre-defined business rules. This paper introduces a novel approach of optimizing process portfolio both locally and globally by considering inter-operating relationships between process models as well as taking into account the minimal change strategy.