Ever since that back and forth about "East Coast being kind vs West Coast being nice" thing a while back[1], I think it's important to distinguish the two. Because they are noticeably different (at least to me) and shouldn't be used interchangeably. I want someone to be kind to me in a meeting. I think niceness could seriously inhibit progress. A kind person will tell me that an idea I have won't work, but they'll of…
East Coast "kind" would be to tell the person working for you about their performance issues early, in a constructive feedback manner such that they could course correct or find a role they are better suited for. The hope is they either improve in the role, find a new role internally, or decide to move on before having to stomach a firing.
West Coast nice would be to let the person working for you continue to perform poorly without feedback, being "nice" to them, but privately considering them incompetent. Eventually you will end up firing them without much warning when a cut actually has to be made. Think Amazon PIP.