It doesn't affect the rendering of material that already exists. But when you put up a post, it's all in limbo until the piwik responds. Granted, "30 seconds" was an exaggeration. I counted this time and it was about 11 seconds.
Slow piwik is actually something I'm accustomed to in a different context: My own piwik database lives on a different server than the sites it's tracking, so when I check stats, everything takes forever. But it's funny seeing it at WebmasterWorld :)