Ethan Marcotte, on time- and budget-constrained organizations websites:
Between the urgency of their work and the size of their resources, spending months on a full redesign isn’t something they can afford to do. Given that, a free theme for, say, WordPress can yield a considerable amount of value, especially to budget-constrained organizations. They can launch their redesign more quickly, and continue reaching the people who need their information most.
So Ethan takes a look at a bunch of free themes, so at least a responsible choice can be made there, and finds
the results were surprising: on a 3G connection, the slower themes I tested took anywhere from 45-90 seconds for any content to appear. In other words, the pages took roughly a minute before they were usable.
What I find particularly scary is that these are just empty themes. I usually attribute the slowness of sites in this category (off the shelf, slap-a-CMS on it) to be what happens on top of the theme. Stuff like uploading too many/too large of images and installing a million plugins that load their own set of resources.