I'm not sure what is meant by improving revenue per "X", does it mean to increase the amounts of "X"? Does it mean to increase the revenue made per "X", and if so, why wouldn't you choose time to be the "X". E.g. what would a lemonade store choose as their "X" and why?
In "Revenue per X" the X is usually the thing sold. For a lemonade stand X might be a glass of lemonade, and you might discuss the revenue as "revenue per glass", which would be the price of a glass of lemonade.
As an alternative you might consider "revenue per customer" which for a lemonade stand is every person who visits the stand.
The difference gives rise to two common ways of a company increasing revenue, for a lemonade stand being increasing "revenue per glass" (i.e. raise the price) or "revenue per customer" (persuade customers to buy more than one glass).