CPU 核心翻倍，程序耗时为什么不一定减半？

from fractions import Fraction

single_core_seconds = Fraction(100)
serial_fraction = Fraction(1, 5)

def model_seconds(total, serial, cores):
    if total <= 0 or not 0 <= serial <= 1:
        raise ValueError("positive total and serial fraction in [0, 1] required")
    if type(cores) is not int or cores < 1:
        raise ValueError("cores must be a positive integer")
    return total * (serial + (1 - serial) / cores)

for cores in [1, 2, 4, 8, 16]:
    seconds = model_seconds(single_core_seconds, serial_fraction, cores)
    speedup = single_core_seconds / seconds
    print(f"cores={cores:2d} time={float(seconds):7.3f} s speedup={float(speedup):.3f}x")

if serial_fraction > 0:
    print(f"infinite-core limit={float(1 / serial_fraction):.3f}x")
else:
    print("no finite serial-part limit in this model")