Grid Computing is becoming a prominent field of interest for distributed systems. The capability to support resource sharing between different organizations and the high level of performance are noteworthy features. However, to define, deploy and execute Grid Applications require significant design effort and complex resource coordination, which can be alleviated by using Workflow management systems. This technology is more and more adopted in distributed systems and Grids since it ensures easiness and flexibility at design-time and performance and dynamicity at run-time. This paper gives a contribution to performance improvements of workflow by presenting a technique to exploit fine-grained concurrency at run-time. The technique is implemented in a workflow enactment service that dynamically optimizes process execution limiting the design effort of application developers.