TL;DR: How I improved the performance of case-insensitive XPath queries by 30–35%, reducing an 8× performance hit to only 4.5–5×. This was originally posted to the SimplePie NG blog.
Parse-at-all-costs Most feeds are a mess. The old SimplePie “OG” took a parse-at-all-costs philosophy, and could handle many of the most broken feeds you could find — at a cost. While the early versions of SimplePie supported the letter of the RSS 2.0 specification, there were a surprising number of feeds which didn’t.