%X In PEPA, the calculation of the transitions enabled by a process accounts for a large part of the time for the state space exploration of the underlying Markov chain. Unlike other approaches based on recursion, we present a new technique that is iterative?it traverses the process? binary tree from the sequential components at the leaves up to the root. Empirical results shows that this algorithm is faster than a similar implementation employing recursion in Java. Finally, a study on the user-perceived performance compares our algorithm with those of other existing tools (ipc/Hydra and the PEPA Workbench). %L eprints2555 %D 2007 %A Mirco Tribastone %C Edinburgh (UK) %B Proceedings of the 6th Workshop on Process Algebra and Stochastically Timed Activities (PASTA) %T Bottom-up beats top-down hands down %P 97-104 %I AESOP %O 6th Workshop on Process Algebra and Stochastically Timed Activities (PASTA) held in Edindurgh (UK), July 2007