Running a large-scale HLA-based simulation in a distributed environment may require large amounts of computing resources at geographically different locations. A simulation running for a long period of time may result in imbalance of load levels at different computing hosts, hence leading to degradation of performance. Performance typically improves if the workload can be equally distributed among the different computing hosts. However, the Run Time Infrastructure (RTI) does not have any facilities to perform load distribution to alleviate this problem. This paper describes an architecture that provides load distribution services over a multi-threaded RTI to improve the performance of HLA simulations. A new metric for measuring the workload for both optimistic and conservative simulation federates is proposed. Load distribution is achieved through federate migration by introducing mobility and reactivity to each simulation federate.