Large slowdown when adding or removing toplevel title

[agumonkey]

With a document of the form

my title

.... // note, 2 paragraphs and one code block for instance

Editing the '#' on and off will cause ~3 seconds blocking (cpu at 100%, Core 2 Duo laptop). Not on the first deletion though. I assumed it was a remote modification of the list of articles, but couldn't see network requests in Chromium devtools.