計算量 の英訳について質問です。以下の1と2で、より自然な文章はどちらでしょうか?

1a. The numerical cost is more expensive for the second method.
1b. The second method is more numerically expensive.
2a. To reduce the numerical cost, we use the first method.
2b. To reduce the numerical expense, we use the first method.


(元のご質問件名:自然な文章はどちら?)