It depends. Say you spend 8 hours of dev work to save 1 second of processing time per call. It will take 28,800 calls until your time investment pays for.
This assumes the cost of dev time is equal to the cost of CPU time. In some cases the additional speed is going to return more value then the cost of the dev working. And other time the additional value of getting the product to market is going to win out.
This assumes the cost of dev time is equal to the cost of CPU time. In some cases the additional speed is going to return more value then the cost of the dev working. And other time the additional value of getting the product to market is going to win out.