When a Pokemon Mega Evolves, does it gain its stats that same turn?

Question

Just want to know cuz weird things happen...
Like my lucario speed maxed when mega evolved is 355 but for some reason my friends charizard hits first and his speed is 317

Answer 1

In all generations where Mega Evolution exists, all stats except Speed are recalculated and applied the turn a Pokemon Mega Evolves. Between Generations VI and VII, however, the Speed stat functions differently.

Generation VI
Speed modifiers are applied the turn after Mega Evolving. This means a Pokemon retains its base form's Speed on the turn it Mega Evolves. Additionally, Sableye's Prankster will still take effect for the first turn of Mega Evolution despite the Mega form no longer having the Ability.

Generation VII
Speed modifiers are applied on the turn the Pokemon Mega Evolves. Sableye's Prankster no longer gives priority to Mega Sableye's moves on the first turn of Mega Evolution.

Mega Evolution (Bulbapedia)

Comments

Minor nitpick: Speed and ability changes happen immediately, turn order changes happen at the beginning of the next turn. This is why prankster still effectively works (because it changes the turn order at the beginning of the turn, before anyone mega evolves or moves), and moves like gyro ball and electro ball calculate damage using the new speed stat.

Answer 2

The stats do increase, but whoever is faster before Mega Evolution will go first. If a Pokemon Mega Evolves from 85 Spd to 100 Spd, but my opponent's Pokemon is at 86 Spd, it will go first, unless a priority move is used.

Comments

Ahhhh that kinda sucks >.<
But thanks it makes sense now :D