i sleep fine at night with the amount of JS i ship on my personal site lol. i'm sympathetic to alex russell's perf complaints, but JS bulimia isn't healthy either.
For me it’s more about not overloading webmention.io with requests for every single visitor of your site. It seems more respectful to just re-build occasionally and thus cache the webmentions on your side. The performance is secondary to me, though it is an accessibility concern as well.