You mix up archers with cowboys, which had to be fast to shoot someone. An archer uses stealth and strikes without getting noticed. Once he is found out the archer mostly has to move spots and start hiding again. Also speed is irrelevant, because he'd never be in face-to-face combat.
Game-wise it strikes me rather to have a high critical hit ratio, some priority moves that circumvent protect/detect/substitute. No speed needed then, but I'm reaching here and this is basically my wishful thinking.
Eh, this is true in some RPGs but definitely not "pretty much every." In Disgaea Archers have bad Speed Growth but good accuracy and attack growth. Final Fantasy Tactics archers have the same speed, movement, and evasion rates as Knights. It's all over the place in the Fire Emblem series, some have Speed the second fastest growth rate for snipers after skill and some have it as worse than attack. And while it isn't an archer, compare to the X-Com series where snipers have terrible movement rates because they can't move and shoot in the same turn (without perks).
Really the strange thing about Decidueye is the high defenses. Not familiar with high defenses for basically any sniper in any game.
I'm not talking about the archer's moving speed, i'm talking about the attacking speed. Not only the archer's attacking speed, but also the arrow's which is supposed to be fast. The archer needs to be fast in order to strike before being discovered and the arrow needs to be fast in order to strike the target before the target moves himself from the perfect spot.
The fact that archers moved from place once they were discovered only makes the fact that Decidueye is going to attack after the opponent attacks him even more weird.