  The process of cosmic ray acceleration up to energies in excess of $10^{20}$
eV at relativistic shock waves with large Lorentz factors, $\Gamma \gg 1$
requires $\sim \Gamma^2$ particle energy gains at single reflections from the
shock (cf. Gallant & Achterberg 1999). In the present comment, applying
numerical simulations we address an efficiency problem arising for such models.
The actual efficiency of the acceleration process is expected to be
substantially lower than the estimates of previous authors.
