From MozillaZine Knowledge Base

"Why it's slower than the competition". The "it's slow because it's XUL" is not correct and it doesn't help promoting Mozilla-as-platform. Unless you have a really old computer, Thunderbird will not be slow because of XUL. After all, Firefox is written in XUL and one of its selling points is that it is faster. --asqueella 09:18, 26 December 2005 (UTC)