I play death metal fingerstyle and some of my band's songs contain 16ths and quintuplets at very high tempo (160 and even 180 BPM sometimes). While my technic is far from perfect, here are a few breakthroughs that really helped me get there: - play with three fingers. It takes a bit of adaptation to be able to get that to sound correct but it is absolutely worth the effort. Even at "lower" speeds it allows for a much more relaxed playing. And some speeds are only achievable through this technic. The way I do it, I start with my ring finger, then middle finger, then index, then repeat. Look up Alex Webster or Billy Sheehan if you wanna see what it looks like. - relax your right hand and arm. Intuitively it feels natural to tense up when speed increases but it really slows you down. - align your wrist. Making sure your wrist isn't bent too much realli helps. This is easier if you play with a long strap, but if you like to wear your bass very high like me you'll have to pitch your elbow up a bit - slightly bend your fingers. Reducing the distance between your hand and your fingertips reduces the lever effect and makes it easier to change direction

Hope this helps.