First, it's important to understand how turns work in FFT. Everyone has a counter that goes from 0 to 100. if someone's counter is at or above 100 they get a turn. If no one is at 100 then everyone adds their speed value to their counter and you check again to see if anyone is at or above 100. When you take a turn your counter gets reset based on the action taken. If you pass completely your counter goes to 40. If you move or take an action but not both your counter goes to 20. If you move and act your counter goes to 0. Note that you don't get to keep any extra above 100 so if your counter was at 99 and you gain 6 it'll go to 105 and then get reset to 0 when you take a full turn. So there is rounding involved which can impact how good specific amounts of speed might be. Also, generally speaking, one of the big flaws in the AI is it likes to move and act even when it doesn't need to. It should skip moving a lot of the time to get reset to 20 instead of to 0 but it doesn't because it's bad. *sigh*
At any rate, pretend I can walk up and punch the enemy right now or I can stand still and use tailwind. What are the ongoing game states, assuming I started with 6 speed? With my counter at 0 it'll take 17 ticks to get another turn. With my counter at 20 and 7 speed it'll take 12 ticks to get another turn. Then assuming I have to move to punch going forward I'm looking at 15 ticks with 7 speed so I gain 2 more ticks every action. So after 6 full rounds of combat I've caught up to status quo and start profiting after that point. That doesn't actually seem terribly good, but one thing to keep in mind when moving faster is sometimes you get to take 2 turns in a row. Not only is this great for tempo it also means you don't need to take the move action on that round, gaining an extra 3 ticks right there.
If you happen to have good ranged attacks and never plan on moving then you're looking at 14 ticks to get a turn with 6 speed and 12 ticks to get a turn with 7 speed. That's still a gain of 2 ticks per action.
What if I don't want to move on my first turn? Compare taking no action at all to using tailwind. With no action at all I get to go again after 10 ticks. With just tailwind I get to go again after 12 ticks, plus I'm saving 2 ticks per turn. This is obviously awesome. Ramza should never skip his action phase. If he has nothing better to do he should tailwind himself (or someone else I guess) instead of waiting.
How about even higher amounts of speed? Here's the tick gain from each additional point of speed based on a full turn or a half turn:
|New Speed||Full Turn||Half Turn|
Note that because the base number of ticks per action is dropping the time it takes for an investment to pay off does drop as well. If you're taking full turns it's pretty big to go from 9 to 10 speed. Instead of taking 12 ticks per action you drop down to 10 ticks per action and it only takes 5 turns to make up that gap. If you don't move on that one turn it only takes 8 ticks to get the action back and therefore it only costs 4 turns to make up the gap.
In the long run this is certainly awesome, but do we actually have a long run? I feel like the answer for most fights is going to be no. Giving up a useful action in order to tailwind is going to be wrong during most fights. Use it when you have nothing better to do (and if you're trying to power level you can absolutely just stand around spamming tailwind for tons of xp/jp) but don't give up a good attack to use it.
Getting speed from other sources is silly good though. I'd skipped getting the speed hat for my mages because I wanted to keep an extra spell power and some more max mana but I now think that's wrong. I should go buy more hats when I get to a store that sells them. And when I eventually find a store selling the accessory that adds speed you better believe my whole team is getting a set of those. Acting more frequently than your opponent is awesome, especially when you're smart and don't move unless you have to and they move all the time because they can.