Light ships have speed = 10, heavies = 5, and galleys = 8. So theoretically a fleet of light ships would cover less trade area in a given month if they were escorted. I seem to recall an earlier version of EU IV (at least a couple of years ago...) showed a penalty to escorting light ships (lower trade income) but last I checked, it appeared to have no effect. Since I just keep my heavies and galleys in port (in peacetime), it would make sense to escort as long as there's no penalty to income. Anybody understand how this game mechanic is handled?
Not a direct penalty but it does lose you money you shouldn't be losing.
Warships should not be unmothballed unless you're about to go to war or feel you're at risk of being declared on.
They add nothing to a trade fleet at all but by having them unmoth-balled you're paying half their upkeep for nothing which especially where heavies are concerned can completely overwhelm what you get from protecting trade.
And during war-time your whole fleet should be together until you've destroyed all the enemy ships and even then, blockading usually brings in more money then protecting trade does until you no longer need the light ships to help blockcade and can send them back to protecting trade.
And the speed of a light-speed is largely meaningless, as once the fleet reaches the trade node it's always considered protecting trade there until you tell it to do something else, no matter where it actually is (if you're at the edge or beyond your supply range your ship will occassionally need to head to port to repair, which doesn't affect your trade income since it's still considered protecting the node. But if you manually tell the ship to do something else, it needs to return to the node before the protect trade bonus kicks back in.
Speed matters more for exploration. And only because of the attrition, they can go farther and make it back more reliably if the fleet is only light ships. Or in the case of El Dorado where you don't give a crap about attrition while on the explore mission, it at least helps you explore a little faster.
Fleets move at the speed of the slowest ship, so mixed fleets also don't matter.