Do you “have” to fire anybody?
You should “fix” (and probably fire, if no choice) the person that breaks the team integrity – this is the only consideration I would suggest taking. A slow performing person is not a critical component in case the rest of the team does not see it this way.
The team is your most valuable asset.
If you have no choice and somebody has to go, following the rule of “make no harm to the team” may bring you to a decision to fire another person, not especially a badly performing or a least popular one.