Unless the user has some capacity or ability to endure the resistant forces they are exposed to by high speed, such as Enhaced/Supernatural Durability or Acceleration Immunity, they will be limited in how far they can increase the striking power before breaking down.
Narukami Sakaki (Absolute Duo)
Asura (Asura's Wrath)
Various Characters (Baki the Grappler)
Kevin Levin (Ben 10)
XLR8 (Ben 10)
Ultimate Way Big (Ben 10)
Kensei Muguruma (Bleach)
Yoruichi Shihoin (Bleach)
Tohka Todo (Chivalry of a Failed Knight); via Raikiri
Speedsters (DC Comics)
Kryptonians (DC Comics); via yellow sun radiation
Martians (DC Comics)
Slade Wilson/Deathstroke (DC Comics)
Midnighter (DC Comics/Wildstorm)
Minatsuki Takami (Deadman Wonderland); via Whip Wing
Akaza (Kimetsu no Yaiba)
Diddy Kong (Donkey Kong)
Various Characters (Dragon Ball series)
Midare Kojima (Grenadier)
Jotaro Kujo (JoJo's Bizarre Adventure); via Star Platinum
Jean Pierre Polnareff (JoJo's Bizarre Adventure Part III: Stardust Crusaders); via Silver Chariot
Kensei (Bleach) can punch at extremely high speed.
Yoruichi Shihoin (Bleach) can unleashed a barrage of ultra-high-speed punches to overpower Aizen.
Jotaro Kujo's Stand, Star Platinum (JoJo's Bizarre Adventure)
Jean Pierre Polnareff's Stand, Silver Chariot (JoJo's Bizarre Adventure Part III: Stardust Crusaders)...
...could attack enemies with barrages of sword thrusts within a second.
Agatsuma Zenitsu's Breath of Thunder - First Style: Thunderclap and Flash (Demon Slayer: Kimetsu no Yaiba)
Bellamy (One Piece) using his Bane Bane no Mi powers to boost his punching strength with the momentum.
The Jet Dials that Gedatsu (One Piece) has attached to his elbows allow him to strike his enemies at blurring speeds. The only downside is the impact creates holes in his clothing.
Monkey D. Luffy's (One Piece) Gear Second technique involves him increasing the flow of blood in his body. By doing so he can then use the Rokushiki/Six Powers technique, Soru/Shave and strike his opponents at super sonic speeds.
Brook (One Piece) using his signature speed strike technique, Hanauta Sancho: Yahazu Giri/Three-Verse Humming: Arrow-Notch Slash.
Kizaru (One Piece) using his Pika Pika no Mi to increase the speed of his kick to build relativistic mass.
Shiliew's (One Piece) tremendous speed allows him to cut down several people at once in a split second.
...he is able to slice his opponents at an incredibly high speed.
Tohka Todo's (Rakudai Kishi no Cavalry) nigh-invincible battōjutsu technique, Raikiri infused with lightning, grants her strike speed which transcends the speed of sound, releasing blinding light and huge amount of heat in the process.