Pliable energy-storage devices have attracted great attention recently due to their important roles in rapid-growing wearable/implantable electronic systems among which yarn-shaped supercapacitors (YSCs) are promising candidates since they exhibit great design versatility with tunable sizes and shapes. However, existing challenges of YSCs include an inferior power output and poor performance consistency as compared to their planar counterparts, mainly due to their unique linear geometry and curved interfaces. Here, a YSC comprising wet-spun fibers of reduced graphene oxide and MXene sheets is demonstrated, which exhibits prominent decreases in the equivalent series resistance and thus increases in the power output upon increasing the length, which is contradictory to the common expectations of a typical YSC, showing revolutionary promises for practical applications. A much higher power density (2502.6 μW cm-2) can be achieved at an average energy density of 27.1 μWh cm-2 (linearly, 510.9 μW cm-1 at 5.5 μWh cm-1) via our unique dual-core design. The YSCs also present good stability upon stretching and bending, compatible with further textile processing. This work provides new insights into the fabrication of textile-based energy-storage devices for real-world applications.