How did Thundurus-I outspeed my Mega Beedrill?

Question

So I'm messing around with a new OU team on Pokemon Showdown, almost at 1200. I had just lost Staraptor to a Thundurus-I after I realized to late that it was a physical Thundurus with Defiant, not Prankster (and had used Intimidate and Defog). However I thought it was at low enough HP to KO with my Mega-Beedrill who had been sitting in the back, but I never got the chance to see because Thundurus outsped me, knocking me out w/ Wild Charge. I'm probably looking past something really simple, but I'm very confused as to how Beedrill did not outspeed Thundurus. Please help if you can :)

Facts:
My Mega-Beedrill's speed stat is 427
Thundurus-I's max speed is 353
He was not Choice Scarfed, as he had used Knock Off the turn prior.
Trick Room/Tailwind was not active.
He was not holding the Quick Claw.
My moves were U-Turn, Poison Jab, Knock Off, and Drill Run.
His moves were Wild Charge, Knock Off, Hidden Power (prob Ice), and the fourth was unrevealed.

I won the battle overall, my Gardevoir outspeeding and KOing w/ Moonblast.

Answer 1

Was Beedrill Mega Evolved when you sent it out? If not, that's your answer. On the turn a Pokemon Mega Evolves, it retains its original form's Speed stat. Its Speed will change to that of its Mega form after the first turn passes. Since Beedrill is not a fast Pokemon in its standard form, it's perfectly reasonable for Thundurus-I to outspeed it before it gets the Speed buff from its Mega form.

For these reasons, it is not uncommon to find Mega Beedrill sets using Protect as one of their moves. The move can be used to stall out the first turn with low Speed, if need be. Protect also has utility outside of getting the Speed boost safely. It allows it to scout for priority attacks, certain moves and more, which can be very useful considering Beedrill's frailty. If you are not already using Protect on your Beedrill, I highly recommend using it.

Comments

That sounds about right. I don't know why I've only found this out now, after M.E. being out for almost two years now :P
Thanks :)

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No problem!