I've spent yesterday by hunting the slowdown, I've rewrote lot of titlesec in custom .4ht file, but still haven't found the cause. There was lot of cases where token registers or \edef commands which were feeded with some new content in each section, but it is still slowing down. So it might be really easier to just contact Javier :)

I am not sure what to do with this. titlesec really can't work with tex4ht, as sectioning commands are heavily redefined by tex4ht and they should be configured using CSS, titlesec's configurations can't be used due to totally different display models between DVI/PDF and CSS.

It would be best if titlesec wasn't loaded at all with tex4ht, only dummy definitions of macros which may be used in the document. The problem is that tex4ht redefines macros after the end of preamble, so we can't fix the macros which were called in the preamble. So you must either modify your document to include problematic package only when tex4ht isn't called, or the package itself must be modified to cooperate with tex4ht.