Any speed differences between the various permalink structures is negligible. I mean, we’re talking likely milliseconds of difference. Forget speed, just go with the structure that makes the most sense semantically for your site. On another note, it makes sense to go with a structure that will make the most sense for your users. For example, it makes sense to go with /category/post_name if you have a lot of categories/posts, as it makes navigation easier for users.
