The neutrons used here have a maxwellian energy distribution associated with fission. A single bit will contain neutrons with energies from nearly thermal up to a few MeV. These neutrons are not relativistic at all. For reference a 1 MeV neutron travels a meter in about 70 ns. This means the spread in the neutron spectrum and the transmission distance will conspire to place a fundamental limit on the data rate. Can't have neutrons from neighboring bits mixing together too much. This doesn't even include intermediate scattering which will make things harder.