Can someone explain to me what makes Deoxys-Speed so threatening?
I would look back at ALL of my posts consisting of reasons why Deoxys-S is S-Rank and why it is one of the most difficult Pokemon to directly handle. Many people haven't, so...let's begin.
Suicide hazard/dual screen lead sets aren't very useful if the opponent has a defogger.
I doubt a defogger is going to switch into Deoxys-S at first. You have to significantly consider this: You don't know what set it will run. Oh, so it's the lead? That means NOTHING! It could be the Anti-Lead. Example: That Tyranitar of yours? This Superpower takes care of it and now you're 6-5 *claps*. You can't know what set it will run unless you scout it. It's quite possibly the most unpredictible Pokemon right now. Why would you put your defogger out there immediately if you don't know what move it's going to use? Suicide Hazard remains its most effective set, yes, and it could fake you out with an Ice Beam (instead of Fire Punch if the team in question has Genesect on lock), scare you off, then proceed to set a Stealth Rock up and Magic Coat your Sableye's Will-O-Wisp back at you.
it's not the best supporter due to it's downright awful defensive typing.
Oh, because the Defensive Typing says EVERYTHING. Let's talk about Tyranitar, Reuniclus, Chesnaught, Lapras, and many other Pokemon that have bad typings. You forget to mention the lack of bulk and how Priority is big, but the typing doesn't matter if the Pokemon in question doesn't have the bulk for a Defensive Pokemon (in all honesty, it's like you switched from Speed to Defense Forme; choose a premise and stick to it!).
It's offenses are too mediocre--
I'll stop you right here...
breathe for me...in...and out...
Now...Deoxys-S doesn't have the sheer offenses, but it has Life Orb and a vast Super-Effective Movepool. Fire Punch/Superpower/Ice Beam/Psycho Boost is one of the hardest movepools to switch into. Genesect, Tyranitar, Gliscor, Landorus-T, Dragonite, Salamence, Scizor, Ferrothorn, and even some Pokemon hit neutrally like Physically Defensive Rotom-W, etc. These are some big threats that can't switch into Deoxys-S. Want the calcs? Fine.
20 Atk Life Orb Deoxys-S Fire Punch vs. 0 HP / 0 Def Genesect: 291-343 (102.8 - 121.2%) -- guaranteed OHKO
20 Atk Life Orb Deoxys-S Superpower vs. 252 HP / 0 Def Tyranitar: 406-478 (100.4 - 118.3%) -- guaranteed OHKO
232 SpA Life Orb Deoxys-S Ice Beam vs. 252 HP / 0 SpD Gliscor: 515-608 (145.4 - 171.7%) -- guaranteed OHKO
232 SpA Life Orb Deoxys-S Ice Beam vs. 252 HP / 4 SpD Landorus-T: 484-572 (126.7 - 149.7%) -- guaranteed OHKO
232 SpA Life Orb Deoxys-S Ice Beam vs. 4 HP / 0 SpD Multiscale Dragonite: 203-239 (62.6 - 73.7%) -- guaranteed 2HKO (guys, that's with Multiscale up...)
Don't think I have to post about Salamence if Dragonite is screwed...
20 Atk Life Orb Deoxys-S Fire Punch vs. 252 HP / 0 Def Scizor: 276-328 (80.2 - 95.3%) -- guaranteed 2HKO
Also, going to your "dies from Priority" argument...
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 0 HP / 0 Def Deoxys-S: 177-208 (73.4 - 86.3%) -- guaranteed 2HKO
...too soon?
20 Atk Life Orb Deoxys-S Fire Punch vs. 252 HP / 252+ Def Ferrothorn: 166-198 (47.1 - 56.2%) -- 21.1% chance to 2HKO after Leftovers recovery
Still a hefty chunk of damage off of the most Physically Defensive Spread here.
232 SpA Life Orb Deoxys-S Psycho Boost vs. 252 HP / 4 SpD Rotom-W: 222-263 (73 - 86.5%) -- guaranteed 2HKO after Leftovers recovery
Followed by...
20 Atk Life Orb Deoxys-S Superpower vs. 252 HP / 252+ Def Rotom-W: 75-90 (24.6 - 29.6%) -- possible 5HKO after Leftovers recovery
If the Rotom-W in question isn't dead, then it's entirely crippled.
Of course, this is all considering the spread of the Defenders as well. The big plus in all of this is that Deoxys-S outspeeds all Genesect, minus ExtremeSpeed, which does this:
252 Atk Genesect Extreme Speed vs. 0 HP / 0 Def Deoxys-S: 90-107 (37.3 - 44.3%) -- guaranteed 3HKO
it still gets out-sped by choice scarf users over 103 speed as well as any priority user (unless you're running a physical set with extreme speed- but then you'd have to give up psycho boost).
Okay, so...flee from Priority users and not many people use Scarf anymore (even on Terrakion nowadays, which I haven't seen in a while), so...
Seriously, what set is this thing running in OU that deems it worthy of S-tier?
You won't know until it's too late.
Oh yeah, and you know...fastest Knock Off...