When you use the SNAP option, the compiler generates extra instructions in the prolog and epilog code for every block. These instructions ensure that the run-time traceback messages (produced by PLIDUMP and the SNAP option on an ON statement) include all procedures that were active when the traceback was requested.
A trade-off of using the SNAP option and creating these additional instructions is that it can have a negative impact on the performance of your application. This is especially true for procedures that are called frequently.