Gameplay speed is a feature in all Super Smash Bros. games that increases or decreases the framerate (and subsequently speed) of gameplay. Each iteration of Special Smash has featured a slow and fast mode. There is also an option in Training Mode that allows the player to increase or decrease the speed of the gameplay to one of several presets.
In Super Smash Bros. and Melee, when gameplay is slowed down, the game runs at a visibly slower framerate, and even the Training Mode menu is slowed down. When playing in fast motion, some visual frames are skipped over during gameplay, and menu interactions are also sped up. However, Brawl introduced interpolated animations for 1/2x and 1/4x speed, giving the impression of 60fps gameplay, while calculations are still carried out at the slower framerate. Smash 4 added this feature to 2/3x speed as well, which was carried over into Ultimate.
Other causes of slow motion
In addition to the Special Smash and Training Mode settings, future games would also introduce other sources of slow motion, while fast motion remains exclusive to those modes. These include:
Outside of Training Mode and Special Smash, fast motion is rarely used, however certain Events in Melee use fast motion.
With the addition of the interpolated animations in Brawl, several issues were created. Most notably, inputs on the C-stick tend not to register correctly, which appears to be a consequence of the inputs made by the C-stick being made for only a single frame, and input buffering not activating when inputs are made in interpolated frames. This issue was fixed in Smash 4, but some other issues persisted, and new ones were created:
Oddly, none of these issues appear to apply with slowdown, which has its own set of issues.