> it was too slow, and I got behind.
This is purely a function of practice. 5 years ago, a friend of mine could TeX math notes substantially faster than I could write them by hand, including reasonably complex commutative diagrams, lots of tricky formulae, etc. A big part of TeX speed is setting up a bunch of personal macros for commonly used symbols (e.g. \a instead of \alpha, etc.).
One thing that I have seen others do is to create a kind of footnote system in their laptop document so that any formula or diagram gets a reference number then they quickly grab the pen and paper and jot the formula/diagram down then straight back to the computer. Then after the lecture just digitalise the footnotes. Really depends on how much of the lecture is formulas/diagrams to text I suppose.
I got fast enough at hand notes, and found that if I kept my book open during the lecture, that I could skip note taking on a portion of the lecture if it was covered adequately in the book.