Slowdown is a mechanic introduced in Brawl, where a fighter or group of fighters have all their actions, movements and knockback slowed down, without reducing the rate at which these factors are recalculated. While the ability to reduce the speed of gameplay has been present in every game in the series, most notably through Training Mode, slowing down gameplay in this way also proportionally reduces the rate at which calculations are carried out, in order to maintain consistency with full-speed gameplay.
When a fighter is under the effect of slowdown, all actions will be increased in length proportional to the degree of slowdown, while all sources of motion, including those inflicted by external sources such as knockback, are also slowed down, however certain interactions with a stage, such as through conveyer belts or wind, are not slowed down. Since a fighter's animations are slowed down, but their relevant calculations are not, hitboxes can behave differently when attached to fast-moving limbs. For example, in Smash 4, Link's forward tilt had a blindspot above and in front of him, due to his sword moving past this area in a single frame, resulting in the hitboxes interpolating in a way which does not cover this area. However when under the effect of the Timer, his sword is in this region for multiple frames, and thus the hitboxes can cover this area as the animation would suggest.
In Brawl, slowdown behaved slightly differently, as it did not affect knockback. This resulted in fighters being launched higher than usual, as their falling during knockback would be slowed down, but the knockback itself was not. This was changed in Smash 4, and remains largely unchanged in Ultimate.
In Brawl, slowdown can only be caused by the Timer item, Luigi's Final Smash Negative Zone, and the Shadow Assist Trophy, which slow down the victim(s) to 1/4 of full-speed. Notably, this occurs regardless of what speed the gameplay is set to, and thus they will have no effect at 1/4 speed, and will have a greater effect at 1.5x speed.
In Smash 4, the effect is inflicted in many more circumstances. These include:
In addition, the slowdown now occurs relative to the gameplay speed, and thus will always have the same potency, regardless of gameplay speed.
All previous sources of slowdown return, but can now also be inflicted by the following:
In Brawl, the rate at which looping hitboxes, constantly active hitboxes which can rehit opponents with a set interval defined by the hitbox's script, can rehit opponents was not affected by slowdown. As a result, fighters under the effect of slowdown could hit opponents with looping hitboxes many more times than usual, as the hitbox would be active for much longer, while hitting just as frequently. In Smash 4, this issue was fixed, however for unknown reasons, looping hitboxes will still occasionally hit 1 or more extra times, despite their rehit rate seemingly being slowed down correctly. Furthermore, in Ultimate, Ness' up aerial still appears to rehit at the same, or similar, rate as it would at full speed, resulting in multiple additional hits connecting.
Electric attack hitlag
Slowdown usually does not affect the amount of hitlag suffered by victims. However, in Smash 4, fighters under the affect of slowdown would take additional hitlag from electric attacks which the attacker would not suffer. This made it possible for direct electric attacks to be used to lock opponents into hitlag until the slowdown effect ended.
In Ultimate, however, the opposite is true, and victims will take less hitlag than usual, while the attacker will be in hitlag for the same amount of time as usual.
Reduced launch distance
For unknown reasons, in Ultimate if a fighter sustains knockback while under the effect of slowdown, and the slowdown wears off while the fighter is still in knockback, the opponent will not be launched as far as usual. This can be clearly seen by using the trajectory guide in Training Mode, which will show the fighter not being launched as far as the trajectory guide shows that they should be. In competitive play, this can negatively affect Lucario, Shulk and Bayonetta, as the slowdown from Lucario's Double Team and Shulk's vision, when using the forward attack, will last until shortly after the opponent is launched, and Bayonetta's Witch Time can wear off during knockback.