Hey Spatans.
The idea isn't to say, you must play @ 40bpm for 2 weeks.
If you notice, with alternate picking, you can go crazy fast, as fast as you can, and its pretty simple to work up that kind of speed, just the picking. The problem arises when people cannot control that speed and synchronise their fretting with the picking.
Now you try it. Go as fast as you can, on 1 note. Pretty easy eh?
Then do a simple 3 note repeated pattern, any pattern, on 1 string. As fast as you can. If you notice your picking is not 1-for-1 accurate, you need to slow down.
How slow? 40 bpm is just a suggestion.
Try this. Palm mute your open low e. Chug on it. Go really fast, but
MAINTAIN A STEADY TEMPO. Then go slower...
steady tempo... slower.... you will come to a point where you find your picking hand tenses up just to keep that steady tempo @ a moderate speed.
Now thats the speed you practice at. The point where you start to struggle for control.
Points to note:
1) Picking hand should be relaxed.
2) Picking must be accurate and consistant. Not loud, soft, loud, soft.
3) Tempo tempo tempo!!!
So when you are this speed struggling to maintain consistant picking and tempo, your picking hand WILL tense up, it is normal. The aim is to get your picking hand to relax, and yet maintain the above points.
Hope this makes sense. The idea is control.