Managers have dozens of demands on their time. With respect to which staff they ought spend the majority of their time, conventional wisdom on management says:

Focus on the "top" $10-15\%$ who are high perfomers with good attitudes. It is important to reward your best people or you will lose them!

The concept of categorizing staff into high, medium, or low, is demeaning and even if used only as a loose descriptor is not a useful way to think about this issue.

Focus on the roughly $10-15\%$ who are your "bottom" employees$-$ those with the most challenging attitudes and worst performance. If you can bring them $"$up$"$ everyone will benefit.

Focus on the "middle group" the roughly $60-80\%$ of employees who are typically good performers but who sometimes struggle with consistency or attitude fluctuations. It is important to keep the majority of employees happy and if you "win" this group plus your "top" group, you've done pretty well.