Great advice from Jak, as always, and this is something I’m working on, too. I can play it at roughly 185BPM, so I’m got 25 BPM to go before I can play MoP at tempo.
In addition to relaxation as a critical factor, study your movements and work on ECONOMY OF MOTION. Limit how far you move the hand and wrist for each picking motion. Every tiny little bit of improvement helps.
Finally, as you’re getting to faster tempos, stop counting each hit in your head. Try every four. SERIOUSLY, this is VERY HARD to get your head locked into at first. It helps to slightly accent each note you count. Not enough that a listener can hear it, but you can.
ONE-two-three-four-TWO-two-three-four-THREE-two-three-four-FOUR-two-three-four.
Play all sixteen notes, but only count the ones in all caps. Just “know” that each number is a grouping of four. It’s hard to explain, but your brain can’t consciously count fast enough to play that fast. You have to leave that to your subconscious.
I was never a thrash guy, so I didn’t work on this when I was younger. Boy, do I regret it now, but I’m pushing through the old-man-sauce that replaced my blood and trying to get there. You’ll probably get there before I can. GOOD LUCK!