At the speed they do, It does.
It also takes skill to mash intense solos while keeping the song sounding amazing.
You try covering any song on guitar from any band listed above.
Not hard to play Motorhead, Coheed, or Metallica. Early Metallica was good because it was simple and not over blown, when they switched to blues based metal and started acting likely that made them super complex I got pissed. Hey I can sweep a harmonic minor scale over a blues progression too. I used to play Welcome Home, We Are The Road Crew, Ace Of Spades, One, and Enter Sandman. I used to know the riff to Master Of Puppets.
I have no clue about Slipknot or Gloria. Most metal isn't as fast as you think it is.
Though there might be three chords in a riff. Not likely to get away with three chords in a metal song, even Paranoid or Iron Man have more.
I think Megadeth is better because Mustaine is beast and he can play circles around an other "thrash" metal act. Mustaine is playing power metal shit as far as I'm concerned. Mashing solo in is arpeggios and sweeps, as long as you're in key and can sequence you're solid. Though it takes time to build the speed and technique.