Like many others I struggle with playing fast notes, especially a long run of 16th notes. My fingers tend to get tangled up but my bow just keeps going! Sometimes I get so disgusted and frustrated I just have to laugh!!! I found if you take it in short bites, like a bar or two and start slow to get sure of where your fingers are going and then gradually speed up. You’ll find your fingers end up moving on their own and you hardly know it! It’s amazing when you get it down pat.