Most scientific applications have high degrees of parallelism and thread-level parallel execution appears to be a natural choice for executing these applications on systems composed of SMT processors. Unfortunately, contention for shared resources limits the performance advantages of multithreading on current SMT processors, thus leading to marginal utilization of multiple hardware threads and even slowdown due to multithreading. We show, through a rigorous evaluation with hardware monitoring counters on a real multi-SMT system, that in traditionally scalable parallel applications conflicting resource requirements are - due to the high degree of resource sharing - accountable for deeply suboptimal performance. Motivated by this observation, we investigate the use of alternative forms of multithreaded execution, including adaptive thread throttling and speculative runahead execution, to make better use of the resources of SMT processors. Alongside the evaluation, we propose new methods to integrate these techniques into the same binary to maximize performance on multi- SMT systems. Our study shows that combining adaptive throttling and speculative precomputation with regular thread-level parallelization leads to significant performance improvements in parallel codes which suffer from inter-thread interference and contention on SMTs.