Hm, I must say that most of my larger warships rely more on firepower and defence than speed. Exceptions are met by my custom fast attack cruiser and battlecruiser designs. So basicaly I build both slow, sluggish battle beasts as well as lean attack craft. So the span goes from no extra engines (since star bases in my home systems often provide speed boni anyway) to 5-9 parsecs.
That's what two engines produce at my present tech level (4+4+1)
Freighters are fitted with the fastest available- the safest way for a freighter to stay alive is speed, IMO.
Constructors get somewhat medium speed combos- after all they dissapear after being used and are merely meant to reach the star bases for upgrade purposes. Since I usualy play on tight cluster maps the travel time usualy takes around 2 to 3 turns max. I wouldn't want to spend too much in cosntruction cost for something that will meet its end within a few turns.
Troop Transports: Now here it gets interesting. I use armed transports with a few different outfits. Design A carries 2 troop pods (2000 troops) and enjoys fastest possible engines (after the weapon space is considered, of course).
Design B instead carries 5 troop pods, its weaponry - and whatever space is left is used for engines (you can imagine that does only leave one engine, if any). Both designs are useful, depending on how far and how safe the travel to the enemy target can be carried out. The slow ships usualy need heavy escorts to fend off enemy attacks, but with their 5000 troops they are just amazing in ground attacks!!!
