Hold the pick perpendicular to the guitar when picking gripping it fairly secure otherwise you might lose it when picking fast from string to string.
Lose grip will make it easier this is true but it will lose some clarity on the notes but then again this is not all that necessary if you are going to be tremolo picking which means you would be playing as fast as you possibly can. Thus rhythmic clarity is unimportant.
Now if you are talking about controled fast picking i say keep a firm grip not only will this improve clarity and such but it will sound better and allow you to palm mute and make it sound good.
On another note lose picking is kinda weak for tone and sounds muddy. And in my opinion is the lazy way out.
Hope that helps out any dude.