I wouldn't say Mega Metagross is better. Sure Metagross is faster, but Solgaleo can run items and has arguably better STAB moves and setup, as well as recovery.
Solgaleo @ Normalium Z
Adamant nature
Ability: Full Metal Body
252 Attack, 252 Speed, 4 HP
-Sunsteel Strike
-Splash
-Flame Charge
-Zen Headbutt / Earthquake / Crunch / Stone Edge
Sunsteel Strike is your primary STAB move and is found on pretty much all Solgaleo sets. It hits incredibly hard and ignores Multiscale, Disguise, and Sturdy. Z-Splash is your setup move, giving you 150% more damage in a single turn of setup. Flame Charge is an additional setup opportunity, giving this set a bit of a double-dance kind of effect. After Flame Charge with this investment Solgaleo outruns max speed base 150s like Deoxys, Mega Aerodactyl, and Mega Alakazam. A bit less speed investment and more into bulk is also possible. Finally there's a coverage move. Zen Headbutt is STAB and hits incredibly hard in Psychic Terrain, especially useful if you've got Tapu Lele in your team (which also blocks Marshadow's Shadow Sneak), Earthquake hits opposing Steel types like Dialga and Heatran as well as Aegislash, Solgaleo, Zekrom, Reshiram, and Primal Groudon. Crunch also hits Solgaleo and Aegislash, but also hits Lunala and Giratina. Finally, Stone Edge is an option to hit Ho-Oh and Yveltal hard.
Full Metal Body is your only ability option, but it's pretty good. Blocking stat drops is always nice, especially on physical pokémon like Solgaleo that are otherwise prone to Intimidate. It also prevents Aegislash from dropping your attack with King's Shield, as well as some other stat drops like Snarl, String Shot, and side-effects from moves like Seed Flare, Shadow Ball, Crunch, and Psychic.