Metal rhythm guitar
In metal, the riff is the song. Lead is decoration until the rhythm is tight.
What "tight" really means
A fast but sloppy picking hand turns high gain into sludge. Tightness mostly comes from three things: where your palm mute sits, how consistent your pick attack is, and how well you lock to the click. More distortion isn't on that list.
Start with palm muting
Palm muting and tight rhythmExactly where your palm goes, how hard to press, and why chugs go muddy.
Then build on it
Downpicking enduranceThrash-speed 8ths without burning out.
Syncopation and off-beat chugsRiffs that hit the "ands".
Alternate pickingFor 16th-note riffs and gallops.
Drop tuningsOne-finger power chords and a heavier low end.