Average speed of a Dragon to watch out for

You might be able to take out flying Tiki in one hit, actually. Her res and HP aren't that good, and you can ploy her to reduce her res even more.

Most dragons beside flying Tiki and F.Grima I find have speed in the 25-30 range, some up to maybe 33-34. What I do with Nowi (buff her up to upper 40s) is quite rare; I've never seen it in arena up to the 718 range. The most I saw was a Nowi with maybe 35-36 spd?

Deirdre comes with spd ploy, so you can pretend she has 33 spd unless you need her C-slot to buff the team. If you use swift sparrow I would say you're right on the edge of being able to sweep any dragon you need to, and if she's in a bind then ally support or at most a dual rally (both rally spd/res and spd/def are available in the 4* pool, or 3* pool if you're willing to spend 2k feathers) should get her above the threshold so that you can save yourself the sacred coins and use your seal for more ploying (res ploy, for instance, to increase her chance of OHKO).

I did some more calculating in my head. I think it's not too much to look for OHKO against green dragons. Fae probably has 48-53HP and at least 30 res. M.Grima has 48HP at neutral and 27 res, which is even lower magical bulk. F.Kana does not have impressive res or HP. With death blow you have 53 atk, yes? (Or you have a boon/bane?) That's good up to 79 magical bulk, and if you use res ploy you can gain another 4-5 points. You're right on the edge. Some hone atk, some ally support, green dragon go bye-bye if she's not using warding breath or distant def.

At 5* Lv 40 there are 5 dragons in the 30-34 range (1 Red, 1 Green, 3 Blues) with 5 under that and 2 over that (L!Robin at 35 and 37 being the high on SY!Tiki). Obviously Boons refines and other skills will play a role, but OG Y!Tiki only has 30 base speed at neutral. So 38 Spd will likely handle a majority of them (especially considering most of the speedy ones are Blue).

Honestly, most dragons have their Speed stat below the 38 line, and to be honest, dragons with higher speed potential or base are either Seasonal/Rare, never used, or Blue. So I'd say you're good with just Phantom Speed 3, but I would suggest rethinking her A - Fury would be a good sub, adding a small boost to offense as well as her already massive resistance, along that small speed buff to allow her to block doubles from even Speed focus Nowis if needed.

At the 700-710 score ranges of my experience at the top of my head:

Nowi - 30-35
Fá - Same, I think?
Little Tiki (Summerine) - 35-40 (a lot more if she's +Spd and Spd dumped)
Tiki Grande - Spd is that something you eat
Corrin - Anywhere from 40-45, depending on how heavily merged she is and if she prioritizes Def+Res
Guy Grima - See Tiki Grande
Girl Grima - Usually mid 30s and above, rarely actually 40 (people don't work on her as much)

Most dragons have speed in the 28-35 range, so unless their builds try to focus on spd, you’ll probs be able to cancel their counterattack

My Kana has 42 with summoner support and speed boon, if its important

F!Grima with a speed boon and a base kit will get to 38 speed. She's the fastest Dragon I think.

But a +atk Dierdre with a base kit will kill her in one hit unless she's got resistance boosts coming from somewhere. In 95% of cases you should be good. Adding L&D3 to your A slot might be pretty effective, just to cover the occasional Rally speed or Rally resistance.