HarfBuzz skips ZWJ and few other control code points (like soft hyphen) when applying GSUB lookups to better match the Unicode expected behavior. AFAIK no other commonly used layout engine does that. (View Post)
You are probably pre-maturely optimizing here. Shaping is not usually that slow, even for relatively big paragraphs.Shaping a filewith 20574 words (one per line), using hb-shape utility from HarfBuzz… (View Post)