At slow, manageable tempos, you can afford to use motions that don't scale to fast tempos. If you only ever play "what you can manage" with meticulous, tiny BPM increments, you'll never have to take the leap of faith and most likely will hit a wall, never getting past like 120-130 BPM 16ths comfortably. Don't ask how I know this.
What got me past that point was short bursts at BPMs way past my comfort zone and building synchrony _after_ I stumbled upon more efficient motions that scaled. IIRC, this is what Shawn Lane advocated as well.
I recommend checking out Troy Grady's (Cracking The Code) videos on YouTube if you're interested in guitar speed picking. Troy's content has cleared up many myths with an evidence-based approach and helped me get past the invisible wall. He recently uploaded a video pertaining to this very topic[0].
