Hypothesis.is looks interesting. It’s an annotation engine, which basically means comments wherever people want to put them (doesn’t Medium have something similar?). I don’t know if there’s a way to pre-provision a comment space at the end of a post, or if that would just have to happen organically (if people can comment anywhere, they can comment at the end).
The JS embed is only about 5 KB, but I think it ultimately brings in a bunch of other stuff like Angular, KaTeX, and some of the KaTeX fonts for math or formulas that people want to type. The good news is that KaTeX is much smaller and faster than MathJax.