prelude書き換えてる最中に送信してしまった
x.prelude %{ input = (1..2_000_000).to_a.shuffle }
で

Comparison:
block with size: 12.0 i/s
[]= with size (each): 9.7 i/s - 1.23x slower
push without size: 8.9 i/s - 1.35x slower
[]= with size (each_with_index): 7.6 i/s - 1.58x slower