This shifts the optimum gas speed to lower values and, as if there were a strict educator behind the chromatographer, speeding is punished more strongly when the velocity must be low anyway. Hence, using a column of doubled length requires more than twice as much run time, because the gas velocity must be lower. Hence the plug corresponding to 2 ml in the outlet is only 1 ml and is half as long. To displace 1 ml, half the velocity is required compared to displacing 2 ml at the outlet. Hence optimization must compromize between a low velocity in the inlet and a higher one at the outlet.
Conclusions are against intuition.
The problem resulting from the compressibility of the gas is obviously in the inlet of the long column. Thus the correct conclusion is that optimum velocities are far lower in a compressed gas. GC with vacuum at the outlet, e.g. with GC-MS, is even faster. The most important argument against the use of hydrogen concerns safety. I welcome your feedback.