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.
Instead, slow them down to speed you up.
Review their code, even if it’s been merged already (introduce them to post-commit reviews)! Most of my code is actually reviewed this way — a basic requirement to keep momentum on a team spread over 3 continents. But anyway, ask questions. Ask the why’s and seek to understand. Ask lots of questions.
Whomever seems the nicest or jives with you the best, ask to pair program after lunch or something. You do the typing and let them do the talking. Learn from them, don’t argue or come across as judgy. Just learn.
Eventually, you’ll be a fast one too and one day you’ll get a review on some code you merged a few days ago…