Another slight modification to make this show the effect every time:

    f=$(mktemp) && \
    echo $'#!/bin/sh\necho Hello' > $f && \
    chmod a+x $f && \
    time $f && \
    time $f && \
    echo 'echo Hello2' >> $f && \
    time $f

On my system:

    Hello

    real 0m0.131s
    user 0m0.001s
    sys 0m0.002s
    Hello

    real 0m0.004s
    user 0m0.001s
    sys 0m0.002s
    Hello
    Hello2

    real 0m0.004s
    user 0m0.001s
    sys 0m0.002s