I had considered these points a lot during this design process and had to ask myself on what basis to establish 'fast' as a combat class.
It's important to understand that 'fast' or 'slow' as combat classes is mostly a matter of giving or denying more or less capable promotion lines for withdrawal and pursuit. That's pretty much it.
Other representatives of speed are the movement value of the unit, and the base withdrawal and pursuit values, to which I think you'll see that yes, a bear can easily outrun a human (but will almost always be doing so when chasing that human down as bears are not known to be timid when injured.)
Thus the bear line has a rather good Pursuit value. However, the bear is not a 'fast' unit because it lacks some agility and reaction speed and is a little numb of mind when it does enter into a chase so could be out-strategized by a clever or skilled withdrawing opponent. Thus 'Fast' indicates a mental and physical 'preparedness' to get a fleeing enemy AND to flee if needbe. It indicates reflexes and an ability to convert practice into very valuable improvement to these abilities. A bear CAN chase down prey but is less likely to be well practiced at pursuit as it does so rarely to hunt and commonly only as a 're' action to a hostile situation which doesn't happen that often because most creatures in the wild are going to be careful not to cross a bear.
A Nuclear Cruiser is indeed a faster ship than a Dromon. However, a Nuclear Cruiser is considered to be a little less maneuverable. It's crew is trained less for speed than it is for battle. It could be very good in an outright chase (its base pursuit would keep the dromon from having a chance unless the dromon was VERY well trained and geared for withdrawal which could mean the dromon is good at finding places where the nuclear cruiser couldn't so easily pass as the dromon) but it's role is not as specialized here as destroyers are. Their pursuit is primarily a matter of getting a slightly faster chasing ship to come up first in the stack if Destroyers are used in hit and run attacks making them a Destroyer deterrent.
In fact, if you notice, the naval units may begin to take on a bit more sensical game balance here. Destroyers are rigged to be best do defend against and attack Subs, but can also be used for hit and run to be able to enter combat with battleships and at least survive the battle, thus can 'swarm kill' battleships. Destroyers (vs Subs++, vs Battleships+) also have interception.
Subs are rigged to be best to attack less maneuverable and thus vulnerable to hit and run Battleships and can almost always survive such a battle enough for other subs to finish the job. They are also quite capable of hitting then getting out of fights with the somewhat sluggish to react cruisers but cruisers could get a bit more skin in the game than a battleship due to some natural but minimally improvable pursuit values so they aren't AS vulnerable as battleships. Subs(vs Battleships ++, vs Cruisers +) Subs are also capable of entering enemy territory unseen and some can carry missiles.
Battleships are very powerful and can devastate a Cruiser as Cruisers are somewhat weaker and cannot attempt escape. They are much stronger than Destroyers and will outmuscle the destroyer in all cases but destroyers can often get away from the fight. This doesn't bother the battleship much because it's doing collateral with its attack usually so when it gets the opportunity to attack its the King of the sea. Without help though, it can't even see the sub to attack it. Battleships (vs Cruisers ++, vs Destroyers +) Battleships are also the best at tearing down city defenses.
Cruisers can chase down Destroyers and are stronger than Destroyers so there's little a Destroyer can do to survive them without developing a lot more withdrawal skill. Cruisers also are good at reacting to the battle with a sub while the sub is still present and can give some chase to the sub but usually the sub can still get away and the cruiser doesn't usually have the ability to see it so as to find it after the battle in a one on one fight in the open ocean. Therefore, Cruisers (vs Destroyer ++, vs Subs +). Cruisers can also carry a hefty payload of missiles in most cases.
Thus on this little circle, Destroyers and Subs are Fast units that rely primarily upon speed and maneuverability to come out on top. Cruisers and Battleships are not, even though the Cruiser is good at moving at good speed so is quite capable of giving a straight chase but can be outmaneuvered by a skilled captain in a more agile vessel.
The Cruiser is capable of a good clean charge but the faster it goes, the less capable of turning it becomes. Thus why its 'combat class' is not 'fast'. A dromon can turn on a dime in comparison to a Cruiser, which
could just mean its ability to survive such a chase...
Along this line of thinking, the Cobra is Fast, but that's not due to its ability to strike quickly - it's Fast because venemous creatures tend to want to strike one or two times then retreat to let their venom do its work. So if a Cobra strikes a person, they are pretty much going to thereafter turn around and hightail it. The Cobra might not be a terribly fast snake in comparison to some but it's very clever at keeping to places where it can use concealment to mask the trajectory of its retreat. Cobras don't have to go too far off to be unseen and like most venomous serpents, is a master of sleight of body and can appear to vanish even to an alert eye and finds cracks and pits in the earth to quickly slither into. It doesn't really pursue though... not in its nature as it typically doesn't have to. The poison does its work pretty quickly usually and if the venom of a Cobra can't take something down its probably not a valid food source for the snake and would benefit the serpent to simply enable it to get away from such a big threat. The way the cobra retreats is skilled and such a serpent could develop that skill even more over the course of its life in the wild.
Stamina has been a part of this discussion but it's effects will be coming in the form of another project on the near horizon so it shouldn't be taken into much consideration here aside from being an impression based factor on base withdrawal ability.
A full list of the combat class plans so far (and 'Speed' is one of the categories that has been plotted out now) is available
here.