Slaking won't kill with Extreme Speed, but if you're opponent packs Fake out you're in trouble. There's also the issue of you dealing a measly 40.47 - 47.81% to Slaking in retaliation. Heck even Hi Jump Kick does 70.23 - 82.93%. In a metagame where the revenge killers have attack stats over 1000, Marowak needs to be careful. That being said you could turn Marowak into an amazing Baton Pass counter with some combination of Magic Coat/Nature Power/Taunt/Heart Swap/Copycat/Whirlwind.