The memory grind (Shadowbringers relic) is very, very easy. You just have to join a cluster farm and then memories will start falling out of the sky.
The other three grinds (ARR, HW, and Stormblood) are varying degrees of slog. For ARR you have to do a lot of checklists, like doing specific dungeons. For HW you have to farm Alexander raids and buy stuff with tomestones. For Stormblood you have to slog through the Eureka grind, which no one is doing ATM.