I've written 3 metal rhythm tutorial so far and if you watch them, you'll see that they aren't very high tempo..... wanna why ... no, it's not cause I can't play fast lol. It's because speed comes with time and practice where often complex rhythms takes more patience and coordination.

It is often harder to play a rhythm that moves from quarter notes to 16th to a fast burst of 32th then back to 8th, then to play a steady 16th note riff at high tempo.

It's simply because, like JJ says, those fast rhythm riffs can only be learn by working for a long period of time and practicing accurately and trying to get faster and faster ... no way around that. You have to train your muscle a lot more than playing complex rhythm.

So you could take ANY riff, practice it slow and accurately, then by using a metronome or any tools that will help you keep track of time, bring your tempo faster and faster.

So if you're into those bands, work on alternate picking and accuracy. It's worthless to be fast but inaccurate. Also practice being as close as possible to the strings, the less time you waste in movement, the faster you'll get. This principle also applies to your picking hand, don't move too much, move just enough to pick the note because the further away from the string you end up, the more distance you need to cover to hit the string again. So economy of movement in both hands just be a major concern.