Well I have noticed that some Pokemon in the turn they mega evolve go later than during the rest of the turns. It's not for all of course, but for some depending on the speed of the opponent do. I saw it with my MegaGardevoir, and an Opponent's MegaGardevoir in a seperate battle.
Also some Megas' stats do go down like Garchomp. Arguebly normal Garchomp is better as it can hold another kind of item.
The ones I remember are Garchomp and Ampharos losing a bit of Speed (in exchange for boosts to Sp. Attack and defensive stats respectively). Even Abomasnow gets slower when he Mega Evolves iirc. Funnily enough the Speed is still the same of the non-Mega on the turn of the Mega Evolution, but they gain the Attack/Sp. Attack and Defense/Sp. Defense boosts instantly.
Although the inclusion of what you mentioned is definitely a good thing, it's still not an umbrella mechanic that affects Megalutions in general.
They are just so overpowered it's not funny anymore.
I didn't mind using Mega Charizard X during my X playthrough, he did a really good job I admit. But this time around I'm even wondering if I should use a Mega or forego it for a change, and then again as mentioned some Pokemon are better off in their non-Megas, like Garchomp.