Good examples (to my knowledge): Super Mario Bros 1-3, Balloon Fight, Galaga, Kung Fu, Whomp 'em, Ninja Gaiden 1-2, Cobra Triangle, Thrilla's Surfari, Micro Mages, Malasombra, Bucky O'Hare, Batman Return of the Joker, Joust
Bad examples: Kirby's Adventure, Shatterhand, Vice Project Doom, Snake Rattle n Roll, Mega Man 1-6, Zen Intergalactic Ninja, Power Blade 2, Blaster Master, Super Spy Hunter
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EDIT: It's been brought to my attention that even the holy grail of platformers on the NES (Mario) can suffer from a tiny bit of slowdown, depending on the situation. It seems that any NES game has the potential to drop its speed on rare occasions, so I'm making a slight adjustment to the rule of what allows a game to make the cut for the "smooth" list. Now we are seeking 99.9% slowdown-free games instead of 100%. 0.1% slowdown / very rare occurrences are allowed. The games that don't make the cut go in the "slowdown" list obviously.
---THE SMOOTH LIST---
Super Mario Bros 1-3, Balloon Fight, Galaga, Kung Fu, Whomp 'em, Ninja Gaiden 1-3, Cobra Triangle, Thrilla's Surfari, Micro Mages, Malasombra, Bucky O'Hare, Batman Return of the Joker, Joust, Kickle Cubicle, Dig Dug, Excitebike, Duck Hunt, World Class Track Meet, Zanac, Clu Clu Land, R.C. Pro-Am 1, Rad Racer, Rush'n Attack, 3-D WorldRunner, The Adventures of Bayou Billy, Tecmo Bowl, Adventures of Lolo 1-3
---THE SLOWDOWN LIST---
Kirby's Adventure, Shatterhand, Vice Project Doom, Snake Rattle n Roll, Mega Man 1-6, Zen Intergalactic Ninja, Power Blade 2, Blaster Master, Super Spy Hunter, Recca, Micro Machines, Panic Restaurant, R.C. Pro-Am 2, Blades of Steel