rake assets:precompile is too slow

My website use AngularJs, HAML, coffescript at front end and rails 3.2.13 at backend. Since AngulraJs, it involves a lot of javascript and hence a lot of assets.

So every time I deploy to either staging or production environment rake assets:precompile take too much time(around 16 minutes )(I ran it on already compiled code with no changes).

I have used 'turbo-sprockets-rails3' gem to only recompile changed files, and only compiling once to generate all assets. But rake assets:precompile still takes around 16 minutes alone.

time rake assets:clean RAILS_ENV=staging

real    0m46.418s
user    0m41.340s
sys 0m2.636s

time rake assets:precompile RAILS_ENV=staging

real 13m14.294s

user 13m4.088s

sys 0m9.344s

Please help me reduce assets compilation time. Thanks in advance.