I knew performance was going to be a problem sooner or later and I had already started gathering information well before I actually started coding 1.3.

Yahoo's awesome YSlow extension was more than useful in improving the rendering performance. This tool makes it very easy to find what should be improved. You could also use Google's Page Speed but it's pretty much a ripoff of YSlow and I don't like their advices (adding a class to each <li> element might be more efficient, but i find ol > li to be much more clean and easy to maintain, just like I prefer Java over assembly).

Besides the obvious code improvements like avoiding fetching too much data in an SQL call, I also used quite a bit of patches and plugins to speed PHP and CakePHP (the backend for Demyo):

On average, the software is now more than twice as fast. Under some conditions, some pages are even ten times as fast.