Grassy Terrain regen effect doesn't seem to follow the norms for who's faster and slower.
With most residual effects, speed is taken into consideration. Lefties will regen for the quicker pokemon before the slower pokemon. If two pokemon proc a weather (or terrain) on the same turn, the slower one will override it and get to keep weather up.
Grassy Terrain seems to give the slower pokemon regen first. I don't know if this is how it works in game, but it's tripped me up a few times and caused me to lose Tapu Bulu prematurely (thinking incorrectly that because I got grassy terrain first that I was quicker).
In this clip Tapu Bulu is slower than Tapu Fini, and gets grassy terrain up because of this. Yet when we do damage to eachother, I clearly get regen before he does.
http://replay.pokemonshowdown.com/gen7ou-496441423
With most residual effects, speed is taken into consideration. Lefties will regen for the quicker pokemon before the slower pokemon. If two pokemon proc a weather (or terrain) on the same turn, the slower one will override it and get to keep weather up.
Grassy Terrain seems to give the slower pokemon regen first. I don't know if this is how it works in game, but it's tripped me up a few times and caused me to lose Tapu Bulu prematurely (thinking incorrectly that because I got grassy terrain first that I was quicker).
In this clip Tapu Bulu is slower than Tapu Fini, and gets grassy terrain up because of this. Yet when we do damage to eachother, I clearly get regen before he does.
http://replay.pokemonshowdown.com/gen7ou-496441423