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GENCLS is not a 'real dynamics' model, and its use is discouraged by NERC. So, your case migth not be a dynamic case after all. Are you adding exciters, governors, pss models as well?
Check the posting "Number of Iterations for loadflow on PSS/E" for additional suggestions.
A tested example is the IEEE39 dynamic case posted at "IEEE 39 BUS DYNAMIC SIMULATION IN PSSE"