The huge performance difference would, itself, seem to me to be a
persuasive reason to go to the trouble of adjusting the system to use
mdoc{term,ml}for HTML or ASCII output from -mdoc output, and ditroff
for everything else. There's no mdocps, right?
There's no mdocps right now, nor mdochtml that's consumption-ready.However, mdocterm is just a front-end for libmdoc, whose output is regular: mdoc.3 right now describes it generally, and when I stabilise -column in the next few days, I'll rigorously define the macro ontology.
I don't know postscript, so the next front-end, once mdocterm is pulling its weight and libmdoc output is regular, will likely be mdochtml.