Propagation becomes very slow for non-absorbing observation planes

This is related to issue #534 (closed)

When I use a non-absorbing observation plane, particle propagation gets incredibly slow around the observation plane. It doesn't get stuck, the particles will eventually continue to propagate, but millions of infinitesimally small steps are made before they do.

This is probably the same issue I described in this comment.

I encounter this issue for 1 TeV electron-induced showers, using the corsika.cpp example, but with a particle cut and a propagation threshold of 4 MeV. The observation plane is set at 8 km.