The big problem that carriers face when attempting to be fast is that despite being up to 50% larger, they really have 10% less space available for components other than the required fighter bays (and possibly less, depending on how well the fighter bays you have fit into the required space). On top of that, the demands upon this space are higher, as a ship's speed is linearly dependent upon the fraction of its size dedicated to its thrusters and so a ship which is 50% larger must dedicate 50% more total size to thrusters, and there is also the issue of requiring 50% more space for its life support and habitation modules. Obviously, a carrier can save some space by cutting the armament (which furthermore frees up reactor output to power the additional engines, which can mean that the carrier will not need more reactors and may need less reactors than the regular ship).
An example to demonstrate the issues of making a carrier fast:
A size-600 carrier must dedicate 240 or more size units to fighter bays. 240 size units translates to 4.8 standard or 5.3 advanced fighter bays; as I obviously cannot use fractional components to fill out the required space, I must dedicate 250 size to standard fighter bays or 270 size to advanced fighter bays on the size-600 carrier, leaving me with 350 size instead of 360 if using standard fighter bays, or 330 size instead of 360 if using advanced fighter bays. Alternatively, I can settle for a size-500 carrier which has 300 space available using standard fighter bays (4 fighter bays required), or I can settle for a size-562 carrier which has 337 space available using advanced fighter bays (5 fighter bays required).
Let's assume that I'm using Ion Thrusters (600 max/520 cruise thrust, 4 max/2 cruise power), have habitation and life support modules with support size 85, chose to go with 5 standard fighter bays on a size-600 carrier (350 space available for non-fighter bay components), and want to achieve a cruise speed of 30. A ship's speed may be computed as
[speed] = ([total thrust] / [total size]) * min(1, [reactor output] / [speed level power requirement])
I will therefore require 18000 total cruise thrust for the size-600 carrier (600 size * 30 cruise speed), meaning that I require 34.6 ion thrusters (this will obviously need to be rounded; 34 ion thrusters gives 29.5 cruise speed for 204 size while 35 gives 30.3 cruise speed for 210 size on a size-600 ship). Let's assume that I settled for 34 ion thrusters (29.5 cruise speed for 204 size). I require 7 life support and habitation modules for the size-600 carrier, which costs a further 21 size units. I therefore need to fit my reactor(s), hyperdrive, maneuvering thrusters, and any other components I wish to add into the 125 remaining space (204 + 7*3 = 225, 350 - 225 = 125).
If we were to strip off the fighter bays and convert this design into a destroyer of equivalent size relative to maximum build size (i.e. a size-400 destroyer), those 34 ion thrusters would propel the destroyer at a cruise speed of 44.2 and we'd require only 5 life support and habitation modules (15 size units), which would leave our destroyer with 181 size units for reactors, a hyperdrive, maneuvering thrusters, weapons, and any other components I wish to add. We could strip off as many as 11 of the ion thrusters and still have a speed greater than that of the carrier (23 ion thrusters on a size-400 ship gives 29.9 cruise speed) while freeing up an additional 66 size units for non-drive components.
It's true that you can switch out the ion thrusters for better thrusters to improve the carrier's speed, but this helps the smaller ship in a similar manner and can increase the absolute speed difference (though not the relative speed difference) between ships of differing sizes which carry the same number of the same quality drive thrusters. (Why can rather than does? Because I'm not sure exactly what you'll consider a 'better' thruster - switching out the ion thrusters for the same size in proton thrusters, for example, will reduce the cruise speeds of the ships but increase the sprint speeds.)
< Message edited by Aeson -- 6/4/2015 6:08:00 PM >