Could make transports move like construction trucks, half their attack range.
You might still get the same time from end to end, just means that you plan you advance so that you can move the last 6 sectors then attack with your troops. You may have to advance only half the day before to ensure that you aren't a sitting duck.

I am not so sure about that idea due to sheer size of the map. I really think half is a bit drastic of a reduction. In real life the support ships keep up with large attack crafts quite handily.
If the pic loads. that was a photo shot of my first battle group. When looking at it i was on the submarine on the left.