Many metal rhythmic figures are rather simple, but they are played at high-speed using downstrokes only. For instance, the classic "master of puppets" is more than 200bpm. I read an interview of kirk hammet saying that that was getting closed to their limit.
I've never really practice speed downstroke picking, maybe i can go up to 160bpm (with eighth note). Not so fast...
And you, what's your limit? how long did it take for you to get there?
Same question for alternate picking: not a specific pattern, but let say standard major scales positions, 16th notes, alternate picking. My (honest) limit is 115-120 bpm. Difficult to get past it. Following some suggestions on this website, I've been trying to make shorter movements, and also to relax (i realized i start to get tense when i reach my limit). I tried to hold the pick differently, with an angle, but I don't really like the sound with an angle. Anyway, I'm working on it.