You can be productive towards packaging potato chips, your job for which you receive a wage, or productive towards writing essays that teach new marxists. They're two very different things (and no shade on chips packaging it's just that I knew someone who did this job and hated it).
As marxists we recognize that capitalism does a lot of things well (but we can do them better). and Lenin recognized this too when he said the USSR had to learn from capitalists as soon as possible. In that moment they learned all about HR, productivity, management, etc. All very capitalist concepts and we could call it Labor resources instead of HR, quotas instead of productivity (I guess), and organization instead of managing, but that would just be semantics. We don't change the nature of things by calling them a different name. Capitalists and liberals are not always wrong just on virtue of being liberals, but they are very tunnel-visioned and mistaken in the cause of things.
What they miss is rest time. You need to take care of yourself too and know your limits, and not only that but enforce them. There are techniques for this. And this goes for organizing too. What they want is to always produce more, but that's not always so easy. One easy way to do this is to squeeze your workers more, but even then they run into limits, technological or otherwise. They want you to always do more at work but they don't give you the (physical) tools to actually save time.
To me productivity is doing the best you can with what you have available. You will always have constraints, they're not a capitalism thing. You don't necessarily always have all the time in the world, or all the knowledge you need for a task. To do revolution you need thousands of people and you might only have 5 right now. Those are constraints and being productive/efficient in this context means to figure out what you can do with 5 people that would be most effective. When you have 10 people, what is the most effective thing you can do with 10 people? Etc. I'm kinda meeting up with the broad definition you gave here. You might like to read some of my design essays: https://en.prolewiki.org/wiki/Comrade:CriticalResist#Design_essays.