Hi everyone,
I think I finally managed to get the tremolo picking technique right. Of course I still need a lot of practice practice but I think I'm doing it in a proper way, using the wrist and all.

I'm not going to ask how to play fast. I actually can play fast and last (still need more practice though, like I wrote before) with a constant speed but only when there is no metronome or song playing. Otherwise I get lost.

For example this week-end I want to practice "No Remorse" by Metallica and there is a riff in that song in which you have to pick the A string fastly. I guess the speed is not a problem but I can't play it on time. And this happens for many riffs I want to play with fast playing. I can't count how many notes I'm playing. I don't know how many notes do I have to play. Therefore I am not in time.
If you can't play in time then speed is a problem.

That's like saying I can drive fast, it's just the corners that make me crash.

Slow down, make the effort to play cleanly, accurately, in time and with both hands in sync. When you can do that consistently you can look at increasing the speed a little.
Actually called Mark!

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Download the tab of the song onto Guitar Pro, and therefore you can establish the tempo of the song; and from there you can put a metronome on and practice the picking with a slower tempo and keep on bumping it up until you get to the correct speed of the song.

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In addition to what Mark said, a massive thing is making sure your downstrokes are on the beat. If you're picking 4 notes to the beat, each beat should start with a downstroke. Start nice and slow, accenting this note each time, and that should help a lot.