Using a nonzero bufsize parameter makes all the difference in the world:

  Using the default (bufsize=0 ==> unbuffered):

    % python popentest.py
    time with os.popen :  0.035032
    time with subprocess.Popen :  1.496455

  Creating the Popen object with bufsize=8192:

    % python popentest.py
    time with os.popen :  0.034642
    time with subprocess.Popen :  0.041376

  Creating the Popen object with bufsize=1 (==> line buffered):

    % python popentest.py
    time with os.popen :  0.034658
    time with subprocess.Popen :  0.04211

Maybe the default for that parameter shouldn't be zero?

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