for actual quantity per pick swing you can't beat the plains. But if you just got the swamp figured out, then you should instead look for large coastal rocks or the big boulders of the dark forest and break those by undermining. I am talking the stuff you can build a house on top of, those giant flat rocks.

the quality of your pick and the damage you do with it from skill drastically improve rock mining. At some point you can break the 'campfire' sized rocks in 1 swing, and running about hitting those becomes very efficient.