Watch the Tremolo Exercise: Fast online guitar lesson by Marcy Marxer from Mandolin for Guitar Players
If you play tremolo faster than your metronome try changing the setting to a slower setting and adding more down and up strokes between clicks. Try slowing down the metronome and playing in 6/8 time. Set your metronome to 72 to get a feel for playing in 6/8. Once you're comfortable keep bumping up the speed until you reach your max. Try this speed picking exercise every day for a couple of weeks or more and your overall playing speed will increase.