The following assumes you did not Sea Lion, and you are a competent player, as are your opponents.
The longer you wait to go into Russia, the more Russians (and probably British) that are there waiting for you. Delaying your advance might sound cheaper, but it is not. You will pay for it later. At best, you’ll break even. At worst, you’ll pay more than you saved.
As Argothair pointed out, if you do take Moscow, your slow units are done. There’s nothing else for them to do in a timeframe that is practical, leading to either a slow loss or an excruciatingly slow victory. Neither sounds appealing to me personally.
If you have slow units, and Moscow is not looking great, ducking south through the caucases and the middle eastern oil fields is made much harder if your force is not highly mobile.
Everything about Germany favors them being fast and flexible the first 5-8 turns of the game. A MIC in Romania is, as you said drummer, 30 IPCs of non-fast units. Could it work? Yeah maybe. Will it work better? Probably not.