You mention 'fast position and chord changes'. My single piece of advice would be to solve this issue by doing things slowly. Slow is the new fast. Slow is the fastest way to do fast. Whatever it is you're wanting to do, do it slowly, preferably with a metronome, or whatever you use for tempo, but set it to slow. Do whatever it is you are wanting to do, slowly, for a while (two, three weeks..? Every day, for ten-fifteen minutes..? Slowly..!). Once it can be done, slowly, well, up the tempo a little. No, just a little (60 bpm becomes 65 bpm, for example...) do this, regularly, for another 'while', and once you've become proficient at it, up the tempo a tad more (65 becomes 70..?). Repeat; if, after a 'while' it's still not smooth, clean, easy, go back down a tad with the tempo and repeat. It is useful, and recommended, to start from the original 'slow' tempo now and again, for a day or so, working back up to one's current tempo, as a refresher. Always with the metronome, or whatever you use for tempo. This is the way to obtain whatever speed one wants, and is the guaranteed fastest way.
Is there a downside..? Yes, it requires a large bucketful of Patience. Those without this essential accessory will not achieve their goal, so be sure to have that bucket, and keep it well topped up. It is of no advantage to do this for hours on end, nor too many times a day, but if time permits, a couple or three sessions of 10-15 minutes (no more...) may be permitted. The key is in the regularity (every day...), in the keeping the tempo slow (hence the metronome or equivalent...) and the Patience.
It cannot, and never does, fail.
Douglas