Mega Evolution Mechanics remain the same from DPP, and keeps the original speed of the mega evolved Pokemon for the first turn. Aditionally, mega evolutions that are generally considered overpowered are banned ( Mega Gengar, Mega Blaziken, Mega Lucario, Mega Kangaskhan, Mega Salamence, Mega Sab...
Only Pokémon check or counter other Pokémon. I haven't explicitly stated that until now, but this is where another common misconception comes in. The following three sentences do not make sense. "Stealth Rock counters Talonflame. Lucario is countered by Earthquake. Rocky Helmet and Iron Barbs ...