Here the result of the benchmark :
user system total real
Iteration n: 1
Rails 4.1.8 : 1 relation 0.490000 0.020000 0.510000 ( 0.531127)
Rails 4.1.8 : 2 relations 0.530000 0.020000 0.550000 ( 0.554869)
Rails 4.1.8 : 1 nested relation 0.570000 0.020000 0.590000 ( 0.607984)
Iteration n: 2