Actually, 70 base Speed is surprisingly good for a Trick Room Pokemon. Let's take Cresselia for example, a Pokémon used and known for Trick Room. Cresselia has a base Speed of 85, 15 more than Skarmory. Skarmory underspeeeds key threats such as Primal Groudon and Kyogre. This allows it to pull of quick Roosts and Drill Pecks. It also has access to Whirlwind, which can render Xerneas nearly useless. It has a collosal 140 Defense, which laughs at Mega Kangaskhan and Mawile. However, all is not well. Skarmory has a pathetic 65 HP, and a decent 70. Shocking this flying armoury won't go down well.
Skarmory @ Rocky Helmet
Nature: Relaxed / Impish (Relaxed is better for TR)
EVs: 252 HP / 252 Def / 4 Att
- Drill Peck
- Iron Head
Maximises bulk and hits hard with Iron Head + Drill Peck. Whirlwind is used to switch Xerneas after Geomancy, or you can deal nuclear amounts of damage with Iron Head