Speed is the cheap part of thrash. The work is keeping a backbeat that still cracks on 2 and 4 while the feet cover every 8th underneath and the hand never stops. The vocabulary is small, and it belongs to Dave Lombardo's Slayer records and Lars Ulrich's Metallica ones. These four patterns sit at ♩ = 140 to ♩ = 170, and the records go faster, with Reign in Blood closer to ♩ = 200.
Exercise 1 is the template, and you have played it before. It is the figure that closed the eighth-note double bass lesson, moved from ♩ = 100 to ♩ = 160: hi-hat on all eight 8th notes, snare on 2 and 4, eight kicks alternating R L R L R L R L underneath. A gap of 60 beats per minute is the whole difference between the two, which sounds modest until you count what each limb is doing.
The arithmetic makes it less frightening. Two feet split those 8ths, so neither foot plays 8ths on its own. Lead with the right and each foot is running plain quarter notes, the right on the four beats and the left on the four &s, which is 160 strokes a minute per foot while the hat hand carries all 320. Your right foot lands with the snare on 2 and 4. Build it in this order: feet alone until both sides weigh the same, then the hat, then the snare, and if the 8ths are not even at ♩ = 120 they will not become even at ♩ = 160.
With a hand and a foot on every 8th, the bar has no gaps left in it, and the first casualty is usually the backbeat. The hat climbs in volume to keep pace with the feet and the snare stops being the loudest thing in the room. Keep the hat strokes small and let 2 and 4 stay on top.
Bell on the quarters, ride body in between
Exercises 2 and 3 move the right hand from the hi-hat to the ride. Snare stays on 2 and 4 and the eight kicks stay underneath, so the new information is the accent. Accented notes are bell strokes, unaccented ones are softer strokes on the ride body, and if a listener cannot tell which is which, the exercise has done nothing. Exercise 2 accents all four quarters at ♩ = 150 for the ringing pulse Ulrich puts under Metallica's choruses. Exercise 3 accents 1 and 3 only, and it runs at ♩ = 170, the fastest tempo in the lesson.
Dropping the accent on 2 and 4 is a mixing decision as much as a drumming one. Those beats already carry the snare, so a bell stroke on top competes with the backbeat instead of framing it, and accenting 1 and 3 alone leaves the bar alternating bell, snare, bell, snare across the four quarters. One accent moved, and the section reads differently. In both exercises the hands land together on 2 and 4, ride and snare; flam them and the backbeat goes soft.
Keeping the gallop long, short, short
Exercise 4 is the thrash gallop, the one pattern here whose feet are not evenly spaced. The hands hold 8th-note hi-hat with the snare on 2 and 4, but every beat of the kick reads 1 & a: an 8th note then two 16ths, twelve kicks in the bar instead of eight. Alternate straight through and the sticking is R · L R on beat 1, L · R L on beat 2; twelve is even, so the bar comes around without a reset. To start every beat on your lead foot instead, double the lead: R · R L, where the repeat falls an 8th apart, the widest gap in the beat.
Long, short, short is the shape, and it flattens into a triplet the moment the third kick drifts early. Check it against your hands. The first two kicks of each beat land with a hi-hat stroke; the third lands alone, one 16th before the next downbeat. Exercise 4 is the slowest pattern in the lesson at ♩ = 140, and those two 16ths are still the fastest notes in it. The naked one has nothing to hide behind.
All four patterns give out in the feet before the hands. Record 30 seconds of Exercise 1 and listen to the kicks by themselves. A loud-soft pulse alternating through them is your weaker foot, and the answer to it is a slower tempo, not more effort.