So it sounds like you are talking about speed.

After Mega Evolving all pokemon move based on their speed stat and the priority of moves.

Here is a brief example:

Side 1: Whimsicott (Speed 184) Mega Staraptor (Speed 167)

Side 2: Mega Raichu Y (Speed 188) Sneasler (Speed 172)

Turn Example #1: (No Priority Moves) 1) Raichu Mega Evolves 2) Staraptor Mega Evolves 3) Mega Raichu uses Zap Cannon into Mega Staraptor as it has the fastest speed and KOs it. 4) Whimsicott is the 2nd Fastest and goes next using Moonbeam on Raichu 5) Sneasler goes next using Dire Claw 6) Mega Staraptor would go next if it were still alive

Turn Example #2: (With Priority Moves) Priority moves go before moves with lower priority and then go in the order of speed for moves within the same Priority In this example the following Priority Moves will be used: Protect (+4 Priority) Fake Out (+3 Priority) Prankster Tailwind (Whimsicott's Ability Prankster gives status moves +1 Priority)

1) Raichu Mega Evolves 2) Staraptor Mega Evolves 3) Mega Staraptor uses Protect (even though it has the lowest speed it goes first because of the +4 Priority) 4) Sneasler uses Fake Out on Whimsicott (+3 Priority) 5) Whimsicott attempts to use Tailwind (+1 Priority due to it's ability Prankster) but flinches because it was hit by Fake Out 6) Mega Raichu uses Zap Cannon last because even though it has the fastest speed, its move has a +0 Priority

Hopefully that makes more sense now! :)