PR: Expand concurrency to benefit from parallelism

time (local)
real	0m0.441s
user	0m0.436s
sys	0m0.000s

time (production server)
real	0m0.900s
user	0m0.888s
sys	0m0.012s

sha1sum old.css 
```d8fd8d435bcef7b85693d4959e22e73b7761e0cc```  old.css

----------------

time (local)
real	0m0.172s
user	0m0.672s
sys	0m0.004s

time (production server)
real	0m0.265s
user	0m0.896s
sys	0m0.036s

sha1sum new.css 
```d8fd8d435bcef7b85693d4959e22e73b7761e0cc```  new.css