Hi,

We are seeing severe performance degradation when the --compress option is being used, in terms of "compilation speed". With the following command line:

$ time node_modules/gulp-uglify/node_modules/uglify-js/bin/uglifyjs --compress --mangle -- ./ui/vendor/jquery-1*.js ./ui/vendor/lodash-*.js ./ui/vendor/jquery.*.js ./ui/vendor/jszip.js ./ui/vendor/ember-1*.js ./ui/vendor/loader-*.js ./ui/vendor/ember-resolver-*.js ./ui/vendor/bootstrap-datepicker.js ./ui/vendor/**/*.js

...

real    2m48.377s
user    2m33.172s
sys 0m4.416s

If I disable the --compress option and only use --mangle, things are reasonably fast (5s for this input set). If I disable both of them, it's blistering fast - less than a second.

Total length of the .js files being minified in this case is around 118k lines.

Any suggestions? Using uglify-js 2.6.0 at the moment, running on node v4.1.1 (but have seen similar speeds elsewhere, in Travis builds).