I have just replicated, then cured, the process I described above.
I created an xhtml file with 70+ explicit sigil Chapter Breaks, entered the Metadata, approved the default TOC (70+ <h2> tags), and then pressed the 'Split on Chapter Breaks' button in Tools.
Attempted the compile. Result - hanging.
Closed Sigil. Started again.
Same xhtml file exactly. Same as above, except this time I did a CLT A to the whole source file in Code View to bring it all looking green before I did the 'Split on Chapter Breaks' thing.
This time it compiled in seconds!
Phew!!
This was a relatively short file BTW. The epub is only 83kb.
I need to work on this a bit more. I deliberately left out any CSS file, and the only tags used in the whole thing were <h1> <h2> <p> and the <hr class="sigilChapterBreak" />
In due course I may be able to add more.
John