Closure compiler advanced mode is an optimizing compiler that will perform several nontrivial transformations while also making your source smaller. Uglify is a minifier, and just makes your source smaller.<p>Apples and oranges comparison. Of course the technology that does more will be slower, especially when the &#x27;more&#x27; requires a great deal of analysis on the input.<p>Note: I&#x27;m not saying closure compiler is better, I actually tend to think that it is not for most usages, as the language it compiles is &#x27;pretty much but not really javascript&#x27;, and the difference there makes it hard to add into an existing build process with confidence.