I have the metronome set on 100 BPM. I am going to play the C major scale straight up and down, one note on every beat. You can start at 100 BPM and see if that tempo works for you. If it doesn't then stop playing! Lower the tempo and try again. Lower to 90 BPM. If that doesn't work go down again to 80 BPM. The point is that you should lower the tempo until you can play smoothly and consistently along with every note and beat.

If you have to slow down to 40 BPM, then that's what you have to do. If you still can't keep up with 40 BPM, don't let this discourage you. Simply turn off te metronome conpletely and work on improving playing the scale. Simply focus on the physical motions of getting your fingers on the notes and picking them slowly and consistently. The more you practice, the better you will get. When you can manage it, then play along with the metronome at 40 BPM.

Whatever speed you are abel to start at, eventually you should attempt to play a little bit faster. Gradually raise the tempo; try raising it one beat per minute until you can't play along. When you find that tempo, you've found your current speed limit. Now you have a target speed to work on improving to!