This is a weird thread without real test data. There's a lot of subjective info that is tough to scale. Most people's experiences are of a limited range and are unable to comprehend the breadth of what's actually possible. It's like if you owned a Geo Metro, a Honda Civic, and a Hundyai Accent and someone asked you to describe how high horsepower feels in a car. You can't. If your mix of experiences included a Corvette, Viper, pickup with a 427, super charged Mustang, etc. then you have a better sense of the raw, effortless force that exists when there is real power there. Anyone's understanding of something comes from their perceived experiences. How fast is something? How detailed is something? How dynamic is something? Well, for a given user, it's dependent on their experiences.
A lot of "speed" music may not be as demanding as you think. Speed for me is more so techno where beats, articulations, and dyanmic ranges can be unnaturally fast. Slower earphones, less articulate/textured earphones, and earphones of lower dynamic range can lose a lot of information, well either not present them or muddy the information together. Sound stage presentation, including space and separation, are part of these aspects. The earphones with better separation, better stage size and depth, are the ones that have both greater and more linear dyanmic range and articulation of note. Some of the faster, more aggressive earphones are more forward, immediate, direct. One thing I like to do is watch music videos when testing earphones. There are a bunch of nice, HD music videos on Youtube. One aspect of a good earphone for me is one that is perceived as very "synced" with the image. The singer is a good source for this perception. Watch their lips move and pay attention to how in sync the audio you hear is from the video. Pay note to variations in the voice, small noises from the lips and mouth. Many, many earphones out there lack the ability to portray many of these subtleties and the ability to present information quick enough to sound "right there" as I'll put it. Some earphones almost sound delayed or you get the sense that you are more an outside observer than right there on stage.
Really, there are a lot of little things that make an earphone great. There are a lot of good earphones out there. There are only a few really stellar ones. Some aspects like speed are subjective. The importance of some of these traits also vary from listener to listener. How fast is fast enough? Depends on the person and what they want from the device. For me, the earphone needs to be fast enough not to muddy the sound during complex information. A simple "noisy" track can show this pretty well. For example, I use Linkin Park as a test because a lot of their music is a lot of noise together, guitars, screaming, drums, ambient noises, and just a mediocre quality recording really. A lot of lesser earphones will have trouble seperating the sounds and making everything in the track unique and separate. A lot of earphones will muddy the information together to some degree. Very good earphones will separate out all the information to where everything sounds separate and in its own place. An earphone like the UM3X is exceptional at this. You will hear everything and everything will be separate.