system
Given the following post, respond as if you're commenting on Reddit:
human
[Superheroes] What’s the lowest sustained max speed required for someone to still be considered a Speedster?

The top human speed is around 27 MPH, which was achieved by Usain Bolt briefly during a sprint. Running at this speed for a sustained amount of time could be considered a superpower, but it would count more as a stamina power rather than speed. So a sustained running speed somewhere between 27 MPH and C is the cutoff point for someone to be considered a Speedster.

What is this speed?
gpt
I think the real question you should be asking is “who and what counts as a speedster?” Because in a lot of worlds, simply being above-average in speed to the average human or being faster than sound still wouldn’t count someone as a speedster in that world. 

Genos from One Punch is one of the fastest characters but isn’t seen as a speedster at all. Simply being as fast as 28 MPH in the One Punch Man world is seen as absolutely not noteworthy at all. 

It would be good to look at some examples and see what type of criteria to narrow down the speedster concept at the core.

*Classification*

- In the DC world, Superman has super speed comparable to some speedsters. He is not classified as a speedster. Because in this world, speedsters are characters tied to the Speed Force (basically a cosmic speed energy.)
- So being called a speedster would vary and need to fit the world’s criteria. Them having it as their major trait or falling under a category. 

*Differences in speed comparatively* 

- In the Sonic world, almost all major characters are extremely fast. Far beyond that of normal humans and non-humans like the anthropomorphic animals in Sonic Forces. That said, characters like Knuckles and Tails while fast aren’t considered speedsters. 
- Characters like Sonic and Shadow are considered speedsters however. Due to them being leagues above everyone in speed comparatively.

*Speed celling and floor* 

Of course, how fast someone would need to be different by the series. 

- Kizaru from One Piece is the fastest character in the world thus far. Being as fast as light, that’s the speed celling. Now as far as the floor for worthwhile mentioning is “faster than the human eye.” This is the bare minimum of what is expected. 
- So for someone to considered a speedster, their speed would need to be above the speed floor and somewhat comparable to the celling. Of course as the story progresses, will see even faster characters and the metrics will be used to find a general idea.  

TL;DR - Being a speedster is more about how it’s portrayed and seen in each world then a concrete number. Varies but the general consensus is that character must at least above normal human speeds at a minimum. As well as need to have speed be a major trait and need to have their speed be fast enough to other fast characters.