Funny thing. As I get really disciplined with the metronome, making sure I only play at speeds I can play relaxed and with minimal errors, I find I have another issue - focus or concentration.
How many times did I just play that A part?
My nose itches.
Ooh ooh ooh, I’m almost through it at this speed, I hope I don’t make any mista… gah!
What time do I work tomorrow?
Whoop, loosen up that shoulder.
I hope I’m not making that weird face I make when I play tough parts.
Crap, did I already play the B part twice?
You get the point. 
My mistakes come less from pushing speed than from my squirrely lack of focus. Sometimes I even over focus. Gotta exercise my attention.
Just thought I’d share. Happy pickin’ y’all!