I've been self-studying a series of textbooks over the past few years (current hs freshman), and have developed a rather effective form of note-taking.
For more theoretical concepts, such as the number theory book that I'm studying for a research project, I like to follow the progression of mathematical logic, and originate everything from axiomatic structure. For instance, if there exist general ideas that are quite trivial and obvious, I would simply denote the concept an "axiom", and then build from that in my note-taking similar to a map so as to reach a certain concept or theorem of which the axiom is a prerequisite to, no matter how distant from the final result. As for the proofs, I usually try to work out elements or claims individually to gather some motivation, and then explain, especially in my own words, why a certain idea or step makes sense, whether in a footnote or within the proof itself. (Note that I handwrite my notes)
As for texts that utilise fewer proofs of theorems and focus more on methods and criterions such as calculus (unless it's calculus with some real analysis; for instance Spivak's Calculus), my recommendation for note-taking would be to structure it similarly to the aforementioned structure, but since it's very method-involved, you should provide numerous examples and work them out yourself to ensure that you understand.
Finally, I should note that mathematics notes can be extremely useful, but, if you haven't grasped the concept, they aren't particularly useful. If you've grasped the concept, then the notes become a tool that can be referenced when constructing a proof of an exercise, or the source for a formula that can be used to solve an equation, integral/derivative, or whatever else.
I hope this helps.
Edit: in regard to writing speed, I suppose, if you're a slower handwriter, you should focus more on grasping the concepts in class and writing formulae that are crucial online. Also, if you use a smaller notebook, you can write down key words and words that you may not be familiar with, and then google them later. As for the formatting, you should do the same, but focus more on directing your attention to crucial concepts so you can reference them later on without sifting through pages of notes aimlessly.