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Compared with lock-based synchronization techniques, Software Transactional Memory (STM) can significantly improve the programmability of multithreaded applications. Existing research results have demonstrated through experiments that current STM designs have slower execution speed than the locks. This paper develops a theoretical explanation for the performance difference. In particular, commit-time-locking (CTL) based STMs are analyzed. A queuing theory based statistical model is developed to quantify the performance of lock-based and STM-based schemes. Analytical results obtained from the model are validated by simulations. Our study shows that (1) lock-based synchronization outperforms CTL-based STMs, and (2) when the contention level becomes low, locks and CTL-based STMs exhibit similar performance. Furthermore, we show that the performance of CTL-based STMs is sensitive to the number of threads, transaction issue rate, and bandwidth of the interconnect. Our results are expected to be useful in the early stages of designing parallel programs, especially on the selection of design schemes for STMs.