Speculative thread-level parallelism has been recently proposed as an alternative source of parallelism that can boost the performance for applications where independent threads are hard to find. Several schemes to exploit thread level parallelism have been proposed and significant performance gains have been reported. However, the sources of the performance gains are poorly understood as well as the impact of some design choices. In this work, the advantages of different thread speculation techniques are analyzed as are the impact of some critical issues including the value predictor, the branch predictor, the thread initialization overhead and the connectivity among thread units