252+ Atk Scolipede Megahorn vs. 252 HP / 252+ Def Slowbro: 242-288 (61.4 - 73%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Slowbro Psychic vs. 16 HP / 0 SpD Scolipede: 264-312 (99.6 - 117.7%) -- 93.8% chance to OHKO
Scolipede is faster, but you still have to back Slowbro, especially if Stealth Rock is up.
Because f.DS does not have enough damage calcs.