Good news! Encore has been largely fixed, and other tune-ups are in the works.
So there's definitely one particular lad that stole the spotlight early these past two days, and that's Hawlucha. Thanks to the new seeds mechanic, Unburden users can get an instantaneous boost in Speed, winning just about any Speed war in a hyper offensive matchup. This has settled as the most popular and efficient way to double Speed, as it persists for as long as the mon lasts in battle. This is especially nice for snowballing sets like SD Acrobatics Hawlucha.
Some replays:
http://replay.pokemonshowdown.com/gen7linked-728200171 (roomtour finals)
http://replay.pokemonshowdown.com/gen7linked-728217585 (thanks again volc lol)
Unburden aside, the issue lies clear in Speed. Other effects like weather and terrain (Swift Swim / Surge Surfer) instead sacrifice this permanence in favor of reusability. While weather has immediately fallen in usage due to being outperformed by Unburden, early test tournaments on Dragon Heaven showed it to be just as deadly. Surge Surfer is the runt of the litter, but Alolan Raichu could very well fill the same role with a Nasty Plot set, or provide obscenely fast FakeTurns.
There were a handful of ways to fight this. Strong priority in the form of Mega Pinsir or Talonflame seems good on paper, but in practice, Pinsir doesn't have the firepower to OHKO even a Hawlucha if it has the Defense boost from a seed. Talonflame suffers in a different way, as its priority is extremely difficult to maintain without sacrificing a move in the link for Roost - and pray it doesn't come in on Stealth Rock! Finally, friendly Speed boosting is an option. Speed Boost Sharpedo, however, needs three boosts to outspeed most Unburden users, which requires two or more Protects in a row. And of course, one could simply run their own Unburden setup. Indeed, none of these options were all too favorable.
This all being said, with helpful feedback from the thread and the OM room, I'm going to go ahead and
quickban instant Speed-boosting abilities. This isn't as extreme as with Shared Power; Speed Boost, Steadfast, Rattled etc. will remain valid. This ban consists entirely of Unburden, Surge Surfer, and the weather-boosted Speed abilities. These abilities' capability to boost Speed without spending any turns are what has been a most immediate problem.
This isn't a magical cure-all sadly. There will be new nastiness rising from the ashes, like the Swagger + Thunder Wave, Fake + Out U-turn dangerous duo described by KuraiTenshi26. Dual screens Koko, Paralysis-inducing Prankster users, and sweepers will continue to rise. Very versatile setup moves like Shell Smash will be particularly valuable as a way to boost both Speed and an attacking stat while using an damaging move. Trick Room remains a venerable threat. "Double Dance", not seen too often, will likely also find a niche among those without Shell Smash or Shift Gear. However, overall, this first decision to remove speed-boosting abilities ought to be a good step in the right direction!