I have that problem:
in my large blog very big number of posts. I have a big base of content and import his in blog, aproximyty number of posts: 70 000. When i upload ~40 000 on my local server emulation, with import new post started erorrs.
And all site work slowly. (from a 0,1-1 sec of genereation news main page index.php with ~1000posts to 4-5 seconds and more for big site)
Maybe my loacal preferense not optimal or somethnig - i not profi in this, but my cpu not bad - core2duo/2gb ram.
Ok, question: aproximaly, how many posts in database support wordpress? Maybe if i have more then 30k or 40k i need to try other cms/blog engine?
I search on codex and forum but not find answer.
And if it not wordpress trouble... when i must search solution? :)