Or, feel threatened by them, drive them away and then complain you need more developers. A bigger group of no-troublemakers looks good and you profit from the larger empire.
What exactly is the problem, do the high performers feel entitled to more compensation?
Being a senior developer is not about velocity, it's about making other people better. If they aren't doing this - if they are only doing their own work - they aren't senior.
This is a pretty normal occurrence. Code base knowledge does matter.