rubyのfor文は遅いことが判明。以下ソースと3回実行した平均時間

k = 0
for i in 1..3000
for j in 1..3000
k += 1
end
end
4.914s


k = 0
3000.times do
3000.times do
k += 1
end
end
4.778s


k = 0
i = 0
until i == 3000
j = 0
until j == 3000
k += 1
j += 1
end
i += 1
end
2.975s