Many companies think their employees are being productive for 40 hours a week, while it's actually somewhere between 10 and 20, with the rest being a big void. If you are actually doing 40 hours a week you are productive as hell.
That's assuming that the work you've been set to do is productive, and you're doing it in a productive manner. For instance, I've spent weeks grinding away on features that never get used by customers. Is that productive? I've also spent hours trying to fix a bug because of infrastructure/platform/tooling constraints that means the code/compile/debug cycle is stupidly slow. Is that productive?
I don't have the slightest doubt I could be a lot more productive than I am now while working less hours. But it's often down to factors that are more or less out of my control.