The point I was trying to make is that, in practice, everyone will run at full optimization, since that's the point of something like Closure. It's not gzip where "good enough" exists sometimes. JavaScript compilers are all about saving users time. Because of that, offering a product that breaks deployment cycles becomes a non-starter.

There are, essentially, an infinite number of optimizations you could make to Closure, though probably several thousand are reasonable. Every marginal optimization needs to run though the entire AST and many of them require prior optimizations to be re-run. As 'cromwellian pointed out, the number of passes is the dominating factor in speed. At some point, it's no longer worth it.