First of all there is Speed. Speed allows you to catch the enemy in combat and/or control the engagement distance. So more is better. Speed does however come at a cost. Afterburners are relatively benign just costing a bit of cap and effectively lowering your agility just a teensy bit. Microwarp drives on the other hand give you a massive boost with some real drawbacks (not so much for battleships but for frigates - ouch) in the form of trickier fitting, smaller cap size, massive cap drain while operating, and they turn your ship into a glowy big target by multiplying your sig radius by 5. Overdrives push up your top speed at the expense of cargo space and maneuverability (we'll get to that). Nanofibers reduce your hull weight which affect both your acceleration and top speed using both afterburners and microwarp drives, but not your normal speed, and they cost you structure - making your ship more fragile once you loose your armor. The point is that speed costs one way or the other but it's tactical advantages outweigh it's costs very often.
Agility is basically how fast your ship can accelerate and turn. This affects things like how fast your ship can get to it's maximum speed, how close you can orbit things and how fast you end up going at any particular orbit distance. Your ability to do all these things is determined by your agility rating and your mass. Interestingly Microwarp drives ADD mass to your ship making it more sluggish. Nanofibers REDUCE your mass and since that goes into this equasion this makes your ship more responsive indirectly. Inertial stabs increase your agility directly. This has no effect on your speed, but your acceleration goes up substantialy.
The thing to remember about all of these are the tactical uses of speed and agility. Frigates are blessed in that you can pile on quite a bit of speed without sacrificing too much agility. Battle cruisers on the other hand? "Ah the nanocane - it goes fast - in a straight line." to paraphrase someone in a fleet recently. Now on to the consideration that most blocade runners know intuitively. Zero to warp speed is important. if you are pointing in the right direction and going at least 75% of your maximum speed, you will warp instantly. If either of these two factors is not met, you ship will turn and accelerate towards maximum speed and warp when the conditions are met. This is where acceleration and agility come in. You need to a) get up to speed and b) turn to face withing about a 5 degree cone of where you want to go before the warp drive engagnes.
An interesting aspect of the mechanics of EvE is that if you are pointed in the wrong direction the system will slow you down to make the turn and then re-accelerate you as you get close to the correct orientation. The faster you ship can turn the faster you can get to the acceleration phase. The quicker you can accelerate the faster you hit that 75% mark. Note that this actualy penalizes faster ships. This is because with equal acceleration it will take a faster ship longer to get to the 75% of max speed point.
Generaly speaking you want resonsiveness in scouting and evasive vessels, speed with tacklers and short ranged weapons ships. Snipers realize they are boned if they get caught so tend to go for range and gank and try and avoid getting into a situation where acceleration or top speed will get them out of trouble.
Three things I've observed:
- if you want to not get tackled - your zero to warp time is in a race against their lock you up and scram you time. This tends to apply to the scouting phase of any fleet battle and any traveling you do. Inertial stabs and Nanos are your friend.
- if you want to tackle and/or get within range of the enemy speed is your friend. Even at the expense of some maneuverability.
- If you are trying to warp - make sure your afterburner or mwd are turned off if possible. Having them turned on slows down your 0 to warp time.
- Determine YOUR standard use of the ship you are fitting. Does it need speed or agility?
- Use EFT to determine the optimum mix of Nanofibers/Inertial Stabilizers/Overdrives (or their corresponding rigs) for the desired effect
- Remember you are using module/rig slots for your speed adjustments. Make sure the speed is worth the loss of other functionality (gank/tank).