Dry malt extract (DME) is easiest dissolved in cold water. When it is added to hot water, it has a tendency to clump therefore taking a lot of stirring to dissolve the clumps. So if using DME, dissolve it in cold water first. Then put the boiling pot on the heat source.
On the other hand, liquid malt extract dissolves most easily in hot water. So bring the water to a boil and remove the pot from the heat source prior to adding the liquid malt extract. The boiling pot is removed from the heat source while adding and dissolving the liquid malt extract because it is quite thick and has a tendency to sit at the bottom of the boiling pot and scorch. Once the malt is dissolved, return the boiling pot to the heat source. Liquid malt extract will pour more easily if it is heated by placing the can(s) in a sink of hot water while the pot is coming to a boil. Be sure to use a spatula to get every last drop.