@ xXJashuaXx Is right in one way, Primarina doesn't have a great base speed, sitting at 60. However, let's run the math on Modest, vs Timid, and with/without a Scarf. I'm going to assume for all intents, and purposes, that the Primarina has 252 Evs in Speed, and a 31 Iv also in Speed, and is level 50.
Modest
Base 60.
Add 20 for the 31 iv.
Add 32 for 252 Evs.
112 Speed.
Add a Scarf, add 56 (50% of 112)
168 Speed.
Timid
Base 60.
Plus 6 for beneficial nature. (+10 to base stat, for a beneficial nature)
Add 20 for the 31 iv.
Add 32 for 252 evs.
118 speed.
Add a Scarf, add 59 (50% of 118).
177 Speed.
Let's compare that, to the fastest Pokmon, in Sun/Moon, Phemosa, who has a Base Speed of 150.
Top speed stat, at level 50:
Assuming 252 Evs in Speed, and a 31 iv:
Modest, 202. Affacted by Sticky Web, 133.
Timid, 217. Affected by Sticky Web, 143.
Let's see what Primarina can outspeed, in terms of base stat pokemon, in Gen 6 OU, comparing level 50 Primarina, to other level 50's. Im going to assume they are affected by Sticky Web, and neither pokemon is holding a Scarf. I'll also assume the pokemon, have a 31 iv in speed, and 252 Speed evs (Standard Sweeper sets).
Base 110 Speed Pokemon.
Beneficial Speed nature, 178.
Speed after Sticky Web, 117.
Neutral Speed nature, 162.
Speed after Sticky Web, 106.
Primarina can outspeed, a giant portion of the gen 6 OU tier, with Sticky Web affecting the opposing pokemon, and neither of them having a Scarf. With a fairly decent movepool, it can be a big threat.
If you have team Support, like a Rapid Spin Blocker, or a Pokemon with Gravity, this Primarina set gets a lot better. Remember, you have up to 6 pokemon to use to support each other. That allows creative sets.