The other ultra-fast combo deck is commonly known as “Oops, All Spells," and despite it’s silly-sounding moniker, is essentially what Hermit Druid combo decks are in Legacy (with the Druid himself banned). The two Hermit Druids being Balustrade Spy and Undercity Informer.

The deck aims to get to four mana as fast as possible via the usual fast mana and black Rituals (Dark Ritual, Cabal Ritual) and mill its whole deck over. After this, killing the opponent is pretty elementary: Narcomoebas get flipped over and then they get sacrificed to Dread Return to revive either:

Angel of Glory's Rise, to revive Laboratory Maniac and Azami, Lady of Scrolls. Tap either of our Wizards to draw from an empty deck and win!

Underworld Cerberus, who you can then sacrifice to Cabal Therapy to get back all the milled Spirit Guides, Street Wraiths, Laboratory Maniac, and a

Wild Cantor. Wild Cantor gets cast, filters mana into blue for Laboratory Maniac and Street Wraith is cycled to win!

The deck has a few advantages over Belcher:

Has a lower amount of mana required to instant win (four mana vs. seven).
Can run main deck disruption such as Pact of Negation and Cabal Therapy due to requiring a lower density of mana.
All of its eight win conditions win the game on the spot, compared to Belcher’s backup plan of Empty the Warrens, which can fold to sweepers.
It’s cheaper (money wise) because the main deck lacks Lion's Eye Diamond!

But has some disadvantages:

Often has colored mana problems. Eight of the deck’s starting mana sources do not produce black mana for the Rituals used.
Has a clunky package of Narcomoebas, Bridge from Belows, and Underworld Cerberus/Angel of Glory's Rise for the kill condition, which becomes problematic if drawn in the opening hand and makes the deck lose to graveyard hate.
Loses out on the wishboard and general sideboard utility that Belcher has.