If every pokemon on the field is hit by perish song and every pokemon dies at the same time, the Pokemon that faints last will score the win. Minimum speed can let you get a win through this with some mons at minimum speed that can squeak past another (usually slower) mon that's at max speed. It's basically another way to increase your win percentage. Going before the opponent is also important, so it's a case by case basis. On high ladder you probably won't be able to outslow a Celesteela with a slow Mew or Shaymin since people are wise to the strat, but there's always a chance someone leaves their speed EVs full. If you're already incredibly slow like Mega Sableye, there's no harm in going slower since you aren't going before anyone anyway, and if you're very fast you shouldn't try to make yourself that much slower.
To sum it up this is just to protect yourself from 1 move making you lose and it may not work. I think this is why most people don't value speed that much. I personally run minimum speed on a regular Shaymin and just a -Speed nature on Shaymin Sky for the stat boost. Don't forget about the stats.
The slower mon faints last if Perish Song is called, so if you're the slowest mon you'll faint last, therefore winning the battle.
Also for me, Tinted Lens is one of the best abilities in this format as it invalidates Steel mons like Celesteela, Scizor and Registeel (which are some of the most common mons) as it turns resists into neutral damage. I've had a bunch of success with Weakness Policy Tinted Lens Latios since it's base 130 SpA makes any decently strong Special attack do a good deal of damage even before a boost.
Imo these are some of the best items and abilities available: Items:
Choice Band/Specs (good power but makes multi-turn moves like Solar Beam fail the second turn)
Power items (very niche but they halve speed so super slow mons are guaranteed to move last which is good for Perish shit)
Flash Fire (especially on Mega Scizor)
Scrappy (not as useful as when Pokestar Spirit was around, but still decent for hitting ghosts like Mega Sableye and Dusknoir/clops w stray Normal and Fighting moves)
So basically when it comes to items there's not that much room for creativity, but the available items are very useful so it's not terrible. There's more diversity when it comes to abilities though, so you can experiment with a bunch of different things
Some kind of recent change has made it so that Metronome (and possibly other caller moves like Copycat) calling two/multi turn moves (Shadowforce, ice ball) now fails all the time, instead of just with a choice item. According to big guy asgdf, in the replay logs it showed an error of "no target" but it's still unclear if this is an ingame or PS issue so I want to hopefully get a final answer on this. Just posting it here since the current implementation basically makes choice items have no tradeoff and nerfs all the two turn moves, very big changes for the actual playouts of metronome battles.
Now I personally suspect this commit Make a move fail if the target is the user and it's not self-target did this, mostly because it was very recent and this wasn't happening until recently, Metronome targets the user, moves are failing, and the first thing in there mentions charge moves. Could be wrong though, but from the recent commits, this one really stood out to me in terms of being related to this new behaviour.
EDIT: Disregard that speculation, 100% wrong interaction
This is a youtube video of some guy doing a (hackmon) metronome battle in the real USUM game. It was the first result but fortunately he used the exact thing I was looking for on turn 1.
In 2:31 the guy charges up Solarbeam. At 3:13 the guy uses Solar Beam successfully so PS is wrong on this one. The mons may be hacked but it's more likely this is a ps bug, than it being ingame and someone making a mod to fix this without telling anyone about it.
Shoutouts to greenheroes for getting me interested enough to look further into this.
Once peaked no 2 with a pair of shaymins long ago. After a long fall down the ladder and being driven so far, those days of selective winstreaks and being the best are over now. I've found a new purpose in the existence of antimeta, counterteaming random people when the ladder's quiet and easy to snipe. It's a fine art, but hard to accomplish with Mold Breaker being the only way to get through most of the common meta abilities. Flower Veil is fun for reliability, but there's that greater sense of satisfaction when you find an Imposter team and bring Illusion/Unnerve or see Prankster and bring Dazzling. Truly, this is an oft underlooked factor of the strategy in Metronome Battles.
According to bulbapedia a priority move just has to target one of your guys, and Dazzling then stops the whole move from being executed. It basically ruins pranksters, I guess counters like this (and other stuff like bad dreams vs comatose) could support the idea of banning scouting in a tour even if they'll barely come up. It's a lot easier to just go in with a meta team/team you like and click away anyway.
Also while looking for some other less matchup-based niches, I noticed "No Ability" is actually a choice here. The funny thing is that it inconveniences Trace by stopping it from copying anything, so having "nothing" ends up being slightly more useful than something like Illuminate.