Yes the AsciidoctorJ is pretty fast, moreover if you configure it as suggested on performance section of asciidoctorj (
https://github.com/asciidoctor/asciidoctorj#optimization).
For example rendering asciidoctor documentation it takes 256ms. The only problem is startup time which takes about 1500 ms. But you only have to start it once per application.
So I think you can use it without any problem if this times fits for you.