... one should not necessarily operate. The Tumblr widget needs about 300ms due to its synchronous access to Tumblr, and the typography plugin needs another 300ms. Ouch. Ok, my text just looks typographically crappy, I don't care.
With the cache, I'm currently at xcache - let's see if it produces strange messages like apc. I suspect conflicts between apc and WP Super Cache for my problems, because every time the problem occurred, I also had these strange GC messages from apc in the log. And no messages from PHP, which indicates that nothing was executed by PHP (which also explains the empty output).
There is support for the object cache of xcache in WordPress. Great, you can activate it with a small plugin. I'll save the link, because without this module the blog is faster (and 27 vs 29 DB queries is not exactly a wildly successful operation of the object cache). With the object cache activated, the blog was immediately back in the 700ms+ zone - quite without time-consuming plugins.
For Tumblr, I will now build a JavaScript-Ajax solution that uses the Tumblr API to find my images and push them into the HTML of the page. It's only for the look and then a Tumblr-Connect problem won't be accidentally cached.
But it's a shame about the quite remarkably well-functioning hyphenation from the typography plugin. You can't have everything. Render times in the range of above half a second I find silly. Maybe I'll change my mind again when I've stared at my blog for a while longer.