When people are looking for a wordpress theme they very rarely are looking at the performance of it. The person is looking at how pretty the design is and if it fits their site. There is however a lot more to it then simple the design of the wordpress. A wordpress theme can add a lot of extra CPU and memory usage depending on how it’s designed.
I’m seeing more and more where wordpress themes are not using style.css or at least .css for their style sheets. More and more they are using style.php which in turn simply outputs the style sheet. This sort of setup suddenly requires an extra PHP process every page load thus increased memory as well to compensate. The fact it’s just going to output CSS still does not matter it’s really bad to be doing. This is especially bad when a wordpress blog is running say wp-super-cache they’re probably trying to serve everything via html files. Well that purpose is defeated when every page load PHP is loading up to serve their style sheet. Now as to why more themes are doing this I’m not sure maybe it’s make them look more professional in the eyes of the users I have no idea.
There is one other problem I see quite a bit and that’s extra features included in the theme. For example we’ve had several users use themes with phpthumb included. The theme was using it in several places to make smaller images of portions. Along with that they also did not configure it properly and some bots found it and started using the install to generate thumbnails for other web sites.
That’s just a few examples but I would advise anyone when installing a wordpress theme take a look at what it offers and what it’s doing. If it’s using php files everywhere you might want to see if it’s actually necessary. If they include extra features or include third party scripts you might want to check if they even set it up properly. I would not rely on the theme developers on these sort of things. I’ve seen some free themes not even doing relative paths properly resulting in serving images and such via their own demo of their script. So I really would not trust the theme maker has done a great job with the other aspects of the theme.