Shakuntala Devi calculated so fast through a combination of self-taught mental math techniques, extraordinary pattern recognition, and exceptional practice. She used specialized methods such as the "crisscross method" for multiplication, enabling her to mentally break down and rapidly solve complex arithmetic problems much faster than traditional approaches. Despite having an average IQ, her brain was uniquely adept at processing numbers and mathematical operations in her head at incredible speeds, which she cultivated since childhood through practice and performance. Her skills included rapid mental multiplication of very large numbers (like two 13-digit numbers in 28 seconds), instant extraction of cube roots and higher roots, and quick calendar calculations. These feats were not just raw calculation but also involved clever shortcuts and algorithmic methods she developed and honed herself. She often impressed by performing hundreds of addition and multiplication operations mentally within seconds, a process involving very fast and efficient information processing in her mind. In summary, it was a blend of specialized techniques, intense mental training, and a unique cognitive aptitude for numbers that made Shakuntala Devi so fast at calculations.