Productivity means spending effort where effort changes the outcome.

Effort is only useful where it makes a difference to the result. Much of what fills a day is applied to parts of the work that are already fine, or that nobody will notice, or that would happen anyway, and that effort is genuine and wasted. Knowing where the leverage sits is most of the skill.

With your current project, ask which part actually determines whether it succeeds. Put your best hours there and let the rest be adequate.