What is the primary cause of loss in film capacitors at RF?

In a film capacitor, the conductive plates are typically made of very thin metallic foil or a metalized film layer. At radio frequencies (RF), alternating current tends to flow only along the extreme outer surface of these conductors due to the skin effect. Because the skin depth at high frequencies is extremely shallow, the effective cross-sectional area of the already-thin film plates is drastically reduced. This severely increases the AC resistance of the capacitor's plates, leading to significant ohmic heating and energy loss. While dielectric loss is a prominent factor in many capacitors, in thin film capacitors operating at high radio frequencies, the dominant cause of energy loss is the increased resistance caused by this skin effect.