Alright, I've been meaning to rant about Mega Garchomp SOMEWHERE, but I've never really gotten the chance. This'll be a long post since I have a lot to say, so put in some music or something if you don't want to get really bored.
Simply put, Mega Garchomp is a vastly underrated threat in OU right now. When people look at Mega Garchomp, what do they usually notice first? The Speed drop and opportunity cost right? Alright. First of all. Mega Garchomp is not meant to be a sweeper. There are Pokemon that do this job better. Mega Garchomp is supposed to be a terrifying wall breaker for Sand teams. Mega Garchomp has plenty of Speed for the role it preforms. It outspeeds all relevant walls. By the way, Clefable does not wall it, expecially under Sand. For Unaware Clefable, it 2HKOs without Sand with a small bit of prior damage while avoiding the OHKO from Moonblast in return, and under Sand, well let's just say, no prior damage is needed. For Magic Guard, +2 Earthquake obliterates it.
4+ Atk Sand Force Mega Garchomp Earthquake vs. 252 HP / 252+ Def Clefable in Sand: 216-255 (54.8 - 64.7%) -- guaranteed 2HKO after sandstorm damage and Leftovers recovery
4+ Atk Mega Garchomp Earthquake vs. 252 HP / 252+ Def Clefable: 166-196 (42.1 - 49.7%) -- 28.9% chance to 2HKO after Stealth Rock and Leftovers recovery4+ Atk Mega Garchomp Earthquake vs. 252 HP / 160 Def Clefable: 201-237 (51 - 60.1%) -- 88.3% chance to 2HKO after Leftovers recovery
+2 4+ Atk Mega Garchomp Earthquake vs. 252 HP / 160 Def Clefable: 400-472 (101.5 - 119.7%) -- guaranteed OHKO
People often compare Garchomp to Kyurem-B. Yes, there is a little bit to compare here, but Kyurem has a god-awful typing, a weakness to Stealth Rocks, misses out on Swords Dance, and doesn't have Fire Blast or Earthquake (it has Earth Power, but it isn't STAB and doesn't come off of its Attack stat.) Garchomp sadly takes up your Mega Slot, but I think the advantages it has over Kyurem well warrant its use.
Garchomp is literally walled by nothing. Clefable? Earthquake 2HKOs. Ferrothorn? Skarmory? Fire Blast exists. Gliscor? Draco Meteor. People also look over the huge boost in bulk it gains upon Mega Evolving. I mean, it has no recovery, but it still takes hits pretty well for a wall breaker.
252 SpA Choice Specs Keldeo Hydro Pump vs. 0 HP / 0- SpD Mega Garchomp: 312-367 (87.3 - 102.8%) -- 18.8% chance to OHKO
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 0 HP / 0 Def Mega Garchomp: 189-223 (52.9 - 62.4%) -- guaranteed 2HKO
252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 0 Def Mega Garchomp: 318-374 (89 - 104.7%) -- 31.3% chance to OHKO
252 Atk Landorus-T Earthquake vs. 0 HP / 0 Def Mega Garchomp: 157-186 (43.9 - 52.1%) -- 14.1% chance to 2HKO
Tl;dr: Mega Garchomp is not a sweeper. It's a wall breaker and doesn't need the Speed. Yes, it uses up your Mega Slot, but it has huge advantages over other Pokemon that give you a real reason to use it.
I will add more to this post later.
252+ Atk Mega Garchomp Earthquake vs. 252 HP / 252+ Def Mega Altaria: 148-175 (41.8 - 49.4%) -- guaranteed 3HKO
4+ Atk Mega Garchomp Earthquake vs. 252 HP / 252+ Def Mega Altaria: 127-151 (35.8 - 42.6%) -- guaranteed 3HKO
If it's Adamant 252 attack it's walled by other mons. That's still just one pokemon