We demonstrated a perturbed three-channel waveform synthesizer for efficient isolated attosecond pulse (IAP) generation and characterization at a low repetition rate of 10 Hz. Intense IAPs centered at photon energies of 60 eV (227 as duration) in argon and 107 eV (128 as duration) in neon were generated from an optimized three-channel waveform synthesizer. By introducing a perturbed pulse to the waveform synthesizer, the IAPs can be controlled and characterized rapidly in a few minutes, which is an all-optical method and enables fast feedback for the tunability of the IAP source. The peak power of the soft x-ray IAPs reached subgigawatts, which is higher than that reported in the literature. The work provides a method for efficient generation and characterization of IAPs, which is very helpful for applications in attosecond pump-probe experiments.