I suspect the adage about "a measure which becomes a target ceases to become a good measure" applies here

Part of the reason most workers work a set number of hours rather than negotiating piecework or payment delivered on achieving KPIs or set objectives, after all, is that organizations seldom have a concrete idea of what output they expect over a period of time. (And some of the objectives that are clearly defined and stable over time like "drive this bus route" or "answer calls during this period" actually collapse quite well into working hours)