There is a framework that we used to use for tim classification in manufacturing that was based around adding value to the product - in software that would be coding, testing, etc. Then there is necessary non-value add, like meetings and slack, that are needed (so are work as you say) but don't in themselves make the product directly, so you want to minimize time spent on them while still accomplishing whatever is needed (obviously easier said then done). Same goes for admin stuff. And then there is non-value add, like HN or whatever.

Personally, I think the non-value add category doesn't translate from a factory to coding. If your job is putting boxes on a pallet, when you're browsing HN instead you probably don't do it faster later. When coding, at least for me, I take breaks to think or clear my mind, and I'm not swapping productive coding for online time wasting, browsing HN (or taking a walk or coffee) is a necessary part of the work that improves productivity.

All that to say, meetings and other management stuff can and should be streamlined as much as possible, small breaks, not necessarily. If someone is actually typing code (or an equivalent activity) 25% of their time sitting at the computer, I'd consider that e extremely productive.