Just a quick update:
Thanks to Gothi[c] and
XDebug, we have conclusively identified the problem with the slow posting, and it should be fixed now.
The first clue was what i did myself to improve it, which was remove some custom code that was emailing each new post for archiving.
The connection i dind't make is that when people subscribe for notifications on a thread, this also causes emails to be generated when some threads are posted on, which made posting on those threads slow.
I'll post again as soon as we have the emailing speeds fixed and things should be superfast then.