Slayer was fast as hell throughout their career, and it sounds like we have former drummer Dave Lombardo to thank. In an interview with Stereogum, Lombardo said he was intentionally trying to push the limits of how fast Slayer's music could be. He also mentions that Slayer never used a click track until 2006 with Christ Illusion, and even then it was only for part of one song.
"I think I was at the time. I was pushing the limits. I remember saying to [late Slayer guitarist Jeff] Hanneman, 'It’s gotta be heavy! It’s gotta be faster! Fuckin’ faster! Let’s fuckin’ play hard!' And I think the whole mindset that we had at the time was just to play faster — physically faster, not computerized faster like it is today. You have the help of different software to make you sound like this fucking Big Ben perfect clock.
"Back then, you had to be on point. There was no click track. I had not recorded an album with a click track until probably Christ Illusion. And on Christ Illusion, we only used a click track for part of a song. So you know, things were very primitive at that time. What you heard on those albums, it was real. It was real human playing, without the help of any computer-generated time signatures or anything. So, we were pushing the envelope, and it was intentional."
Lombardo also touched on the double bass drum break in "Angel Of Death", which he said all started because of a live drum solo.
"Well, it was part of a drum solo. They used to let me just go off on the drums. We used to play a song called 'Show No Mercy', and it started with a drum solo. And I remember one particular show, I believe it was in the Valley in LA. I did a drum solo, and then I stopped and I just let the double bass go, and it caught Hanneman’s attention. He went, 'Dude, we should put that in a song." I think at the time we were writing some of the Reign In Blood music, and he said, 'You should put that in the middle of this song.' And so we did.
"Did I know? Huh-uh. No. It was just a really cool surprise little section of the song. You know, you’re listening to the song, and boom, it goes into this drum fill. I thought it was great, but I didn’t know to what extent it was going to impact the listener. And apparently, it was one of the really cool moments of that record."