Lower quality troops do slow games down, no question about it. But I suspect they will also use less ammo rather than more, not only because of the rate of fire but because cowering troops don't shoot at all.
And don't forget about the motivation rating. Unlike CMx1, in CMBN you can have soldiers that are very enthusiastic about their jobs despite being marginally competent at them e.g. early war SS, as well as highly trained troops who don't have a **** left to give e.g. late war British But there does seem to be some overlap, as my casual observation has been that green troops tend to be more fragile than regular troops of the same motivation.