We investigate the impact of fast ions on turbulent transport in Wendelstein 7-X (W7-X) using the gyrokinetic code GENE, building on earlier findings of fast-ion stabilization of ITG turbulence and focusing on hydrogen minority heating in a Helium-4 plasma, motivated by upcoming ion cyclotron resonance frequency (ICRF) experiments at W7-X. Nonlinear simulations show that moderate fast-ion densities and temperature gradients reduce electron and ion heat fluxes by up to 40%–50%. However, beyond a threshold in fast-ion concentration or gradient strength, the fast-ion heat flux increases sharply and becomes a dominant contribution to the total turbulent transport. Linear simulations reveal that this transition is associated with the emergence of a fast-ion-driven mode, localized in regions of unfavorable magnetic curvature. The mode is driven by fast-ion temperature and density gradients, and arises from a resonance involving both binormal magnetic curvature and parallel advection—the latter being important in W7-X but typically negligible in tokamaks. These results demonstrate that fast-ion effects depend sensitively on their drive: while they can effectively stabilize thermal turbulence, sufficiently strong gradients can trigger fast-ion-dominated transport that limits confinement improvement. The findings provide insight into ICRF heating scenarios in W7-X and fast-ion effects on turbulence in optimized stellarators more generally.