Why does Cloyster need more than ~72 EV in Speed (with neutral nature)?
I'm kind of a noob, but here's my thinking. This gives a Speed stat of 194 at Lev. 100. My assumption is that either you got the shell smash off or you're probably slower, so why try to be faster without the smash? With 72 ev and the shell smash +2 Cloyster's speed becomes 388. This out speeds the meta as, for example, 252 Jolly Talonflame has 386 speed. (am I correct that +2 doubles the stat?)
From the Cloyster analysis: "Maximum Speed investment allows Cloyster to outspeed Choice Scarf Latios with Hidden Power Fire and the occasional Modest Venusaur in the sun, among other things."
You are correct that +2 doubles the stat, but maximum speed investment helps it beat scarfers to avoid getting revenged easily. The majority of priority is physical, meaning that Cloyster can weather them with 50/180 physical defense.