Reactions are distinct from speed in many ways, although they are best friends forever. There are plenty of characters who aren't all that impressive in speed, but have enormously fast reactions. This is also what enables characters to trade close range blows at super speed. This is why a character who can run at 100s of mph on a flat desert or straight road may not be able to trade blows or dodge at those speeds. A human may not be fast enough to hang with a werewolf, but they can react fast enough to pull the trigger on their gun and kill it with a silver bullet. Some characters have psychic attack they can use in an instant, but they are bound to a wheelchair, paralyzed.
One of the most important aspects, is that on the same speed tier, someone with faster reactions will appear much faster and has the advantage, being able to precisely counter every single move. This is a tier list of character reaction times.
SOME MILDLY INTERESTING INFO ABOUT REACTION TIMES (Also see here)
For humans, reaction time averages around 200 milliseconds, or a fifth of a second, though people in general vary between poor reactions of 300 milliseconds, and quick responses of 150 milliseconds, depending on the person and the situation.
Human reactions seem to peak around 100 milliseconds, though brain activity tests on sprinters have shown that some level of reaction is possible down to 85 milliseconds. The fastest possible "reaction" in a real world human is the innate reflex, which sends a signal only to the spinal column instead of the brain, allowing you to command a hand to jerk away from a hot kettle in as little as 75 milliseconds. Movements nearer to the spinal nerves can occur in as little as 40 milliseconds (such as in the neck). However, generally these types of "reactions" are not the kinds that enable you to dodge a punch, or catch a falling object, as they are startle and pain responses that are automatic and isolated without a calculation of response, or coordinating enough muscle groups for complex movements. As such, 1/10th of a second can generally be considered the peak of human response in a practical sense, though as previously mentioned, for most people they can react and press a button from responding to a light around 1/5th of a second or less.
1 millisecond: 1000th of a second
1 microsecond: 1,000,000th of a second
1 nanosecond: 1,000,000,000th of a second
1 picosecond: 1,000,000,000,000th of a second
Average human reaction time: 200 milliseconds
Peak human reaction time: 100 milliseconds
Sound Speed: 340m/s per second at sea level and defined atmospheric pressure/temperature (actually 343, but fuck you)
Hypersonic: 5 times the speed of sound, or 1700m/s approx
Normal human/Peak Human perception - 300 to 100 milliseconds - able to react to sound speed attacks from 102 to 34 meters (though it should be noted that real world humans would not be able to see small sound speed projectiles like bullets in flight)
- Humans and most other land mammals
Peak Human/Superhuman perception - 100 to 10 milliseconds (1 to 10 times Peak Human) - able to react to sound speed attacks from 34 to 3.4m/to Hypersonic from 170 to 17 meters
- Master Chief (without Cortana - reacts in 20 milliseconds)
- Spider-man (reactions - 15 times a normal human/13 milliseconds)
Sonic/Supersonic melee - 10 to 1 milliseconds (10-100xPH) - able to react to sound speed attacks from 3.4m to 34cm (react to sound speed melee attacks)/to Hypersonic from 17 to 1.7m
- Kasumi Kenshiro
Hypersonic melee - 1 millisecond to 100 microseconds (100-1KxPH) - able to react to Hypersonic attacks from 1.7m to 17cm (within melee range)/to Mach 100 from 340 to 34m
- Deshwitat L.Rudbich
Massively Hypersonic perception - 100 to 10 microseconds (1K-10KxPH) - able to react to Mach 100 attacks from 34 to 3.4m
Massively Hypersonic melee - 10 to 1 microseconds (10K-100KxPH) - able to react to Mach 100 attacks from 3.4m to 34cm (within melee range)
Nigh Relativistic perception - 1 microsecond to 100 nanoseconds (100K-1MxPH) - able to react to Mach 1000 attacks within "melee range"
Low Relativistic melee - 100 to 10 nanoseconds (1M-10MxPH) - able to react to Mach 10,000 (1%c roughly) attacks within "melee range"
Mid Relativistic melee - 10 to 1 nanoseconds (10M-100MxPH) - able to react to Mach 100,000 (10%c roughly) attacks within "melee range"
- Magneto (17 times a human according to Marvel) (!)
- Professor Xavier
Lightspeed melee - 1 nanosecond to 100 picoseconds (100 Million to 1 Billion times Peak Human) - able to react to lightspeed attacks within "melee range"
- Silver Surfer
FTL melee reactions
- Dio Brando
Massively FTL melee reactions
- The Flash
Infinitely quick reactions
- Giorno Giovanna, Gold Experience Requiem
- Haruka Kaminogi, Dragon Torque
- Combat Speed Fallacy
- Destructive Capacity