Mega Pokemon Question -- speed stats

Hi guys. I’ve been wondering if the mega speed stats work like in Generation 6 or Generation 7. In Gen 6, the Pokemon’s base form speed stat would apply to the turn they mega evolve, but in Gen 7 the mega evolution Pokemon’s speed stat goes right into effect. For example Beedrill (base 75 speed) vs Mega Beedrill (base 145 speed). Would I get that 145 speed the turn I mega evolve or would it be the 2nd turn? I’m doing a nuzlocke and the speed tiers would actually matter

its a gen 6 game so i guess gen 6 mechanics

See that’s what I would guess, but I know that they had done updates beyond the time Gen 7 was out so I wasn’t sure if they applied any of those mechanics