If it feels like you're flicking your leg, chances are something went wonky in the kick mechanics, either that or you're still working to untrain the TKD style round kick where the leg comes up chambered then snaps through the target.
When a Muay Thai round kick is done right, the leg naturally extends and whips through the target, it doesn't feel like you're flicking or snapping your leg to straighten it and drive it through. The video a few posts above of Yodsanklai kicking in slow motion is pretty much textbook on how a kick should work. At the start of the kick, the lower leg gets "left behind" by the hips & upper leg and the knee is bent at around a 90 degree angle. As the momentum & energy transfers through the kinetic chain, the lower leg "catches back up" and the leg starts straightening out as it nears the target. Ideally, you want your hip & thigh to rotate a bit past the target and still have some bend in the knee at the point of impact, this is what gives full power transfer and drive through the target.