Vertical migration is a well known technique to improve the performance of a computer system in which the selected primitives (instruction sequences or functions) are moved to a lower level in the software/firmware hierarchy. Concerning the steps to be performed to apply this technique, we have taken into account in the paper the selection problem of vertical-migration candidates. To solve the selection of these sequences which leads to the highest time saving for control-store filling, a suboptimal algorithm which considers the structural aspects (interrelationships) between the candidate sequences is presented. To validate this algorithm, it has been run over a selected high number of different sets of candidate sequences, and the results obtained show that the average deviation between the suboptimal solution and the optimal one is under 5% and, on the other hand, that the computer time and memory space required to evaluate it are drastically reduced.