Alright, let me start with, I'm glad you are enjoying it, and nothing I say should dissuade you from continuing to enjoy it. I enjoy what I enjoy despite many people's attempts to dissuade me, but music to me is greater than language. And it should be to you.
But...
"...Slayer being played at double speed..."? That's just hyperbole. Have you listened to Slayer? I mean, like, any Slayer?
At 0:50, they speed up. I mean, they weren't playing as fast they could yet, and they still sound like any other band sped up. Most of their albums clock in at less than 40 minutes, and yet they still have more notes than most albums 72 minutes long (not just Godflesh, an analogy I often made when people would complain about albums being short).
Alright, that's not my only criticism, but I'll leave it at that. It's actually really good, once I got my expectations in place. I completely agree with meticulously crafted. A lot of the better blacker metal is "indie", meaning it's recorded in teenagers' basements, so sounds atrocious -- I can actually make everything out. So...yay!
You should also try to find Garden of Shadows. Only two albums from the 90's, but easily the best sounding extreme metal I've heard. I used to know one of the guys (who didn't appear on either album) vaguely, and he said they spent a lot of money on good equipment to make it sound good. Having the overhead on the mixing board and an engineer who knows what he's doing is surprisingly important in "busy" music like this.