Glucose, fructose, saccharose (sucrose), lactose, are sweet. You feel the taste of sugar!
Concentrated sugars are chain of glucose.
Starch, glycogen, other polysaccharides are not sweet but their are concentrated glucose under the form of trains which each wagon is a molecule of glucose.
It is misleading to call concentrated sugars as slow sugars! They represent on the contrary a big carb load for the body metabolism.