Yep, this is more of a sanity check that we are not adding too much overhead. I was also curious about the internals of TeX and where it spends most of its time. And now I finally have a simple way to compile TeX from source 
Just a quick test on a 44 page note that I am working on at the moment, using time:
xelatex lecture.tex 1.51s user 0.04s system 107% cpu 1.450 total
tectonic lecture.tex -r0 1.94s user 0.03s system 99% cpu 1.969 total
(clean directory, 1 pass only, load_fmt_file takes only about 10% of the runtime in this case)
They are basically comparable, so not a big deal at the moment. This is definitely a low priority.
I’ve been using tectonic as my default ‘edit -> compile -> edit’ engine, and I’ve been happy with the performance.