Loose sugar will dissolve faster in liquid than sugar cubes, because more of the surface area of the sugar crystal is exposed to the liquid. To dissolve sugar cubes more quickly, crush them up or break them into small pieces before adding them to your liquid.
Sugar dissolves more quickly in a hot liquid than in a cold liquid, because the molecules of the hot liquid are more active those of the cold liquid. To dissolve your sugar more quickly, heat your liquid in a saucepan or in the microwave before adding the sugar.
Stir the liquid--hot or cold--with a spoon to dissolve the sugar more quickly. Stirring the liquid increases exposure of the surface of the sugar crystals to the abrading effect of the liquid as they pass through it.
If the sugar does not continue to dissolve, add more liquid. Liquid can reach a saturation point, at which there is no more space for the sugar molecules to occupy. Adding more water gives the sugar more room, and it will dissolve.