There is now an option on the fleet movement page to split the fleet in order to reduce detection time at the cost of using more campaign fleets slots.
Example: You send 10 Cruisers and select to split into 5. It will be displayed as a fleet with size 2000 (5), the (5) denominates the quantity it is divided into. Instead of the detection time of a fleet of size 2000, it will have the detection time of a fleet with size 400 (2 Cruisers) and will use 5 campaign fleet spots instead of 1.
After start moving the number of divisions can’t be adjusted, but the fleet will have the option to be merged at any time. You can free up campaign slots with this if needed.