  We examine properties and propagation of the energy-density and
photon-probability centroids of electromagnetic wavepackets in free space. In
the second-order paraxial approximation, both of these centroids propagate with
the same subluminal velocity because of the transverse confinement of the
wavepacket and its diffraction. The tiny difference between the energy and
probability centroid velocities appears only in the forth order. We consider
three types of wavepackets: Gaussian, Bessel, and non-diffracting Bessel. In
all these cases, the subluminal propagation is clearly visible in the intensity
distributions and can be measured experimentally in both classical-light and
single-photon regimes. For Gaussian wavepackets, the half-wavelength delay is
accumulated after propagation over about 12 Rayleigh lengths.
