If we attempt to do to much at once, we shall not do anything properly, because He who begins many things, finishes but few.
In the old days before motor transport, when horses were in great demand, blacksmiths were busy men. The ‘irons in the fire‘ were for forging into horseshoes, and if the blacksmith had a number of horses ready to be shoed, he might try to save time by putting too many irons in the furnace, so that the heat was insufficient to make them all red-hot.