Use multiple buildpacks on your app and get line-by-line timings in the left margin, like this:
2013-10-30T22:39:01+0000 -----> Grails 2.2.0 app detected
2013-10-30T22:39:04+0000 -----> Installing OpenJDK 1.6...done
2013-10-30T22:39:05+0000
2013-10-30T22:39:05+0000 | Loading Grails 2.2.0
...
$ heroku config:add BUILDPACK_URL=https://github.com/ryanbrainard/heroku-buildpack-timings.git
$ cat .buildpacks
https://github.com/heroku/heroku-buildpack-nodejs.git#0198c71daa8
https://github.com/heroku/heroku-buildpack-ruby.git