import pandas as pd
import numpy as np

df = pd.DataFrame({
    'sales': np.random.randint(100, 10000, size=100_000),
    'discount': np.random.uniform(0.0, 0.5, size=100_000)
})

# Approach A
%timeit df.apply(lambda row: row['sales'] * row['discount'], axis=1)

# Approach B
%timeit df['sales'] * df['discount']