Well I could play somewhat fast alternate picking 140-150 bpm 16th notes. But when it comes to fast downstrokes for rhythms and stuff used in Metal, my speed is decent. Like in DT riffs for example, the rhythm parts meant to be played with downstrokes are to fast for me, I can play them alternate picked but I wanna learn the proper way. When I go to fast with my down strokes, my hand stiffens up and mess up. Are there any exercises or ways to improve this? Also is the way pick important, like I see James hetfield palm mute and pick with thumn and index but his other fingers are wide open, when I palm mute though I close mine up a little like a fist as I find it easier to mute the strings. I've tried this hetfield technique and I feel somewhat more comfortable and faster, I dunno. Any input on this is greatly appreciated, thanks.