I think the big stumbling block for many in learning new techniques and/or developing speed is that they work at things they CAN'T play, as opposed to things they CAN play. I'll make sense of that now

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If you practice playing 8th notes at 60 bpm, you should be able to do that with no problem. Practicing THAT, relentlessly, is how you'll build muscle memory and technique. It won't happen though if you try doing the same at a speed you're not capable of doing. I was taught to find a BPM I can play perfectly at, then drop it a couple of BPMs and play it over and over, flawlessly. After doing that for a while, take a break, and raise the BPM. Keep doing that over and over, and then one day ya just wake up able to do the impossible. Muscle memory kicks in and its almost magical.
I did that learning slap bass. Slap was impossible for me, I'd have to do things at ridiculously slow, non musical speeds - but I was relentless. Keeping in time with a metronome was crucial too. You can check my FAIK video in my sig to see how that worked out. I'll add that that's probably the only slap bass I ever did. I never use slap in any bands I play in.
One other thing that really helped me. A good friend told me he practiced bass considering it a meditation. He said he would sit and play an open E for 10 minutes straight, just feeling and meditating on it - connecting as best he possibly could. That turned what once looked like "practice" into a really enjoyable thing for me. I also like trying to express myself in as many ways as possible playing that single note. Change my attack, the way my finger hits the string, how long I let it ring out... you can do a billion things with a single note.
All the above - and time. Though doing the above shortens the amount of time it takes to get better more than any other practice technique I know. And a metronome for me is crucial to it all. Despite what JB thinks

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metronome - Google Search