I wonder if this might be penny wise and pound foolish. Hypothetically, if it takes even a single week longer to make code much more efficient and only 20% of that workforce is driving to work, making coffee, running their laptops, etc, do you come out ahead?
The XKCD comic on these needs a separate column for open source. If thousands of companies are using my library, then there really is a case to be made for making it as efficient as we can within the bounds of maintenance concerns.
For example, a SaaS app with mainly one running instance has a really low multiplier vs. a popular open source project.