I've already tried expensive headphones that are considered fast, like Stax electrostatic headphones. Every headphone I've tried in my entire life has the same speed.

Since I was wondering about these claims about speed in headphones, I decided to EQ a M40x and a Stax 009 simultaneously and trying to feel some difference, I didn't feel any difference. I made adjustments to the EQ using sine sweep, so it was even more similar to each other.

The feeling of separation is not something magical, it is just a correct demonstration of what has already been done in the music. Those who produce the music can slightly separate the sounds at different locations of the projected stereo image of the music.

Many audiophiles claim utopian things about this and end up creating a huge FOMO about this feature, don't think you'll be able to perfectly separate each instrument from an orchestra with dozens of instruments played simultaneously (You will be able to separate a lot of things, that's true, but it won't be absolutely everything). Even if the orchestra is in front of you, it will not be possible to separate perfectly.

The same happens with metal songs, there are songs that are very dirty and that's it, the headphones won't solve the "problem" of the song.

The activity between 100Hz to 1.7KHz and 4KHz to 10KHz are usually regions that, if they are very incorrect, can disturb the feeling of separation/soundstage. In some headphones these regions may have some exaggeration to demonstrate some things in a more distant or more separate way, like on the HD800.

Most planars have correct activity in these regions. Frequency masking/auditory masking is also something you have on some headphones that can get in the way of the feeling of separation and detail, but on planar headphones it's quite rare from what I could tell from my own experience.

Many planar headphones have some little exaggeration in the treble between the 4KHz to 20KHz region, this can cause a feeling of more detail and separation, the DT770 does the same.