>Performing Mega Evolution will not alter the turn order, so a Pokémon with a lower Speed than their opponent will still go after them on the one turn the Mega Evolution occurs, even if their Mega Evolution increases Speed and would otherwise cause them to go first. The opposite is also true for Pokémon who lose Speed from Mega Evolution, such as Garchomp. However, turn order will be determined normally on every turn afterwards.
Same with abilities like prankster. That's why mega Sableye is the best mega of all time.
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