This might have been there previously, but I just noticed it now.
You know the Quick Sum feature. Click at the bottom of or to the right of a column of numbers, click the icon, press Enter or the green check mark, and you get the sum.
Check out how smart it is, though. When you've got some subtotals in the column, it just adds up those numbers.
Now, it only works this way when you have the lines as shown with no empty rows. If you have empty rows, the sum just includes the first subtotal, or the first field period, and you have to update the range to get what you want.
If you do subtotals a lot, check out Data > Subtotals.