Custom Game has changed.

Ladies and gentlemen, a recent change in the code on PS has completely overhauled the way speed works, therefore marking a massive change to the Custom Game "meta". If you are interested in how speed PREVIOUSLY worked, check this link, but remember that it is now officially outdated: https://pastebin.com/CQACFfEB

Thanks to the new speed rules, it is now much easier to understand the workings of speed than it was before.
Now, it is as simple as this:
Speeds 1 to 9999 - Speed works linearly. A Pokémon with a higher speed stat will outspeed a Pokémon with a lower speed stat.
Speeds 10000+ - Two Pokémon with 10000+ speed will speed tie with each other.
Unlike in previous times, Trick Room will now allow Pokémon with speeds lower than 10000 to outspeed Pokémon with speeds of 10000 or higher.

What does this mean?
Well, for starters, this means that every single Pokémon can reach the "highest speed" (note: any value that is 10000 or higher) even without a +speed nature at level 9999, thereby making 9999 the optimal level to use, unless you're using some type of FEAR strategy.
Additionally, Sticky Web is no longer the broken beast that it used to be, as long as your speed remains in the 10000+ zone you're good to go! (Some slower pokemon will still be negatively affected by sticky web, but it's such a small margin that it basically doesn't matter.)
Now that many Pokémon are no longer required to use an Iron Ball just for the sake of reaching the formerly optimal speed stat, you can run good items such as Mental Herb, Focus Sash, Leftovers, Life Orb, whatever you want! It's all within your reach!
Speaking of items, you can now viably run Mega Stones so that you can use some interesting ability combos, such as Drought Houndoom into Solar Power Mega Houndoom. Just remember that you can only mega-evolve once per battle!

Basically, fast Pokémon are no longer suboptimal options just because of their speed stat. Go nuts with your Mega Rayquazas and Mega Mewtwos!

...Just keep in mind that you STILL don't have any reason to be using Deoxys-Speed.