I'm not sure exactly what you mean.. are you asking that if you for example mine 1 iron, 1 coal, 1 iron, 1 coal, and repeat that mining fashion, will it be slower than if you mined all coal? I don't think there is any difference. The only variables to slow mining are your mining level vs. other peoples mining levels, and the type of ore mined vs. your current level. Not including random events. Hope that helps.