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
Rails 4.1.8 : 1 relation 0.520000 0.020000 0.540000 ( 0.566061)
Rails 4.1.8 : 2 relations 0.520000 0.030000 0.550000 ( 0.558834)
Rails 4.1.8 : 1 nested relation 0.620000 0.010000 0.630000 ( 0.648122)
Iteration n: 3
Rails 4.1.8 : 1 relation 0.410000 0.010000 0.420000 ( 0.429014)
Rails 4.1.8 : 2 relations 0.690000 0.010000 0.700000 ( 0.722002)
Rails 4.1.8 : 1 nested relation 0.460000 0.010000 0.470000 ( 0.481997)
Iteration n: 4
Rails 4.1.8 : 1 relation 0.550000 0.010000 0.560000 ( 0.574620)
Rails 4.1.8 : 2 relations 0.440000 0.000000 0.440000 ( 0.450749)
Rails 4.1.8 : 1 nested relation 0.510000 0.010000 0.520000 ( 0.524351)
Iteration n: 5
Rails 4.1.8 : 1 relation 0.510000 0.010000 0.520000 ( 0.539800)
Rails 4.1.8 : 2 relations 0.550000 0.010000 0.560000 ( 0.573357)
Rails 4.1.8 : 1 nested relation 0.520000 0.000000 0.520000 ( 0.532718)
user system total real
Iteration n: 1
Rails 4.2.0 : 1 relation 0.680000 0.040000 0.720000 ( 0.742432)
Rails 4.2.0 : 2 relations 0.860000 0.030000 0.890000 ( 0.908243)
Rails 4.2.0 : 1 nested relation 0.790000 0.000000 0.790000 ( 0.810091)
Iteration n: 2
Rails 4.2.0 : 1 relation 0.630000 0.020000 0.650000 ( 0.659048)
Rails 4.2.0 : 2 relations 0.700000 0.020000 0.720000 ( 0.717271)
Rails 4.2.0 : 1 nested relation 0.950000 0.020000 0.970000 ( 0.983464)
Iteration n: 3
Rails 4.2.0 : 1 relation 0.640000 0.010000 0.650000 ( 0.670463)
Rails 4.2.0 : 2 relations 0.690000 0.000000 0.690000 ( 0.697321)
Rails 4.2.0 : 1 nested relation 0.960000 0.030000 0.990000 ( 1.016978)
Iteration n: 4
Rails 4.2.0 : 1 relation 0.640000 0.010000 0.650000 ( 0.650829)
Rails 4.2.0 : 2 relations 0.830000 0.010000 0.840000 ( 0.868031)
Rails 4.2.0 : 1 nested relation 1.050000 0.020000 1.070000 ( 1.090863)
Iteration n: 5
Rails 4.2.0 : 1 relation 0.710000 0.010000 0.720000 ( 0.737765)
Rails 4.2.0 : 2 relations 0.750000 0.000000 0.750000 ( 0.763060)
Rails 4.2.0 : 1 nested relation 0.670000 0.010000 0.680000 ( 0.692329)