I get why fortress artillery is an immobile or slow moving unit, but why on Earth is militia a slow moving (it takes more than two weeks to move one region on average) is beyond me.
Yes, I know that militias during this time largely stayed in their home area and were useless for offensive actions. Still, the problem is that there are so many brigades which have 2 light or line infantry regiments and a single militia regiment, making the whole brigade pretty much useless. It is even worse when sometimes a two regiment line units gets a militia regiment as reinforcement, de facto stopping any movement for the whole stack (just happened to me) until you notice what is going on and take out the brigade with militia regiment.
Militia regiment are not very effective to start with, so I do not know why they should be a slow moving unit. If you want to stimulate how bad they were, just further lower their effectiveness and cohesion, or at least do not have mixed brigades with line and militia regiments.