btw the difference between Iron Valiant and Darkrai's speeds
Natural Positive nature:
383 | | Dragapult | 142 | 31 | 252 | Neutral | 0 |
381 | | Ribombee | 124 | 31 | 252 | Positive | 0 |
379 | | Meowscarada | 123 | 31 | 252 | Positive | 0 |
377 | | Greninja | 122 | 31 | 252 | Positive | 0 |
376 | | Pelipper | 65 | 31 | 252 | Positive | 1 |
375 | | Zamazenta | 138 | 31 | 252 | Neutral | 0 |
375 | | Tornadus-T | 121 | 31 | 252 | Positive | 0 |
371 | | Barraskewda | 136 | 31 | 252 | Neutral | 0 |
370 | | Cinderace | 119 | 31 | 252 | Positive | 0 |
+1 Positive nature:
574 | | Dragapult | 142 | 31 | 252 | Neutral | 1 |
571 | | Ribombee | 124 | 31 | 252 | Positive | 1 |
568 | | Meowscarada | 123 | 31 | 252 | Positive | 1 |
568 | | Dragonite | 80 | 31 | 252 | Positive | 2 |
565 | | Greninja | 122 | 31 | 252 | Positive | 1 |
562 | | Zamazenta | 138 | 31 | 252 | Neutral | 1 |
560 | | Basculegion | 78 | 31 | 252 | Positive | 2 |
558 | | Revavroom, Moltres-G | 90 | 31 | 252 | Neutral | 2 |
555 | | Cinderace | 119 | 31 | 252 | Positive | 1 |
What this tells me Darkrai's extra speed can outspeed Rimbombee to taunt it or use Sludge Bomb, can outspeed Cinderace/Greninja to attack them on same boosts, can possibly outspeed Adamanant Zamazenta on same boosts to finish off a heavily weaken'd Zamazenta, and of course you also outspeed Iron Valiant with same boosts instead of speed tying it.
Dragonite at +2 is only relevant if Darkrai uses tera to resist or be immune to Extreme Speed, or if Darkrai has Psychic Terrain Support. It is otherwise OHKO'd cleanly by +2 Adamant Extreme Speed and only needs to be chipped to be KO'd by regular E-speed.
+2 252+ Atk Dragonite Extreme Speed vs. 0 HP / 0 Def Darkrai: 214-252 (76.1 - 89.6%) -- guaranteed OHKO after Stealth Rock and 2 layers of Spikes
+2 252+ Atk Tera Normal Dragonite Extreme Speed vs. 0 HP / 0 Def Darkrai: 321-378 (114.2 - 134.5%) -- guaranteed OHKO
You also don't KO Dragonite from full health, even with Ice Beam, so it's not really relevant.
252 SpA Darkrai Ice Beam vs. 0 HP / 0 SpD Multiscale Dragonite: 204-240 (63.1 - 74.3%) -- guaranteed 2HKO
Dragapult with neutral nature will speed tie you, but Dragapult is like 68% using a positive nature, so more often Darkrai will be outsped. Even if you do go face to face vs like Dragon Dance Dragapult, the thing becomes a 50/50 as Dragapult can OHKO Darkrai with +1 Dragon Darts. If neither player has used their Tera, Dragapult is as the advantage as it can survive Darkrai's Ice Beams as Tera Dragon and Sludge Bombs as Tera Fairy.
Everything else is not OU, so its hardly relevant too. By definition, you'll rarely see Meowscarada, Pelipper (much less Scarf Pelipper), Torn-T (unless it gets buffed), Basculegion, and Barraskewda (which outspeeds both in rain anyways).
The speed difference between Darkrai and Iron Valiant is only marginally helpful for Darkrai. You only get to guarantee yourself outspeeding 4 OU Pokemon, 1 of which is kind of a gamble with guessing what move it uses and what item it may have and only if you have 2 specific moves and the other is Iron Valiant itself (who runs Booster Energy like 64% of the time, so you can only do so with Scarf). And then 2 Pokemon you can outspeed or speed tie if they are running neutral nature.
The speed can come in handy, but while also not being the offensive powerhouse that is Iron Valiant. Thing is Darkrai was really scary back in the day because with Nasty Plot it could outspeed most teams and sweep with rarely anything being able to outspeed it. Iron Valiant with Sword Dance or Calm Mind will have similar power while also being at +1 when its first sent out assuming webs are not up (which case not like Darkrai likes Webs either). And people already adapted to Iron Valiant even before Sticky Webs were popular. Because of that, I don't Darkrai is gonna be a problem. It's Iron Valiant but worse in most categories and only slightly better in other 2 categories.