Tire inserts typically increase deformation of the tire because you run less pressure (*typically*). According to Cushcore they improve rolling resistance (I'm skeptical as to how broadly that applies). They also have more damping than air so I could see that hurting rolling resistance in some cases. What I feel is the weight while accelerating though. When I say playing around in the driveway it's not that noticeable I really mean it's not that bad. I can feel the difference in weight in the driveway. You can actually feel it just spinning the wheel by turning the cranks by hand.The insert is a massive change to the structure of the tire. The tire also has less room for deflection to absorb any rocks or roots which may cause losses of energy pushing you upward rather than forming better with the terrain and keeping you rolling forward. You're assuming the weight is the sole reason it might be draining your energy without taking into account the other variables.