The faster you play the more your accuracy counts. Basically because if you miss a note at high speed, you need to play catch up with all the others.

Take note of the BPM where you feel you're loosing tempo. This will become your next goal. You have the right approach where you take it slow and build up the tempo.

What you could is do your normal drills 10 bpm shy of your goal tempo. Once in a while, try out that goal tempo. Don't spend too much time on a tempo you can't handle. That will screw up your technique. By just trying it out once in a while you'll be pushing yourself while keeping your accuracy and technique.