Aside from replacing the battery, which it's not worth doing until it drops to around 80%, there is not much you can do.
It should say underneath the % in the battery health tab that your phone is running at peak performance and if it does, there's nothing to worry about. If it doesn't, there will be a slider that allows you to disable any throttling that is stopping it running at its peak performance.
What makes you believe that this would happen with a loss of just three percent?
You're asking why he thinks apple does sneaky stuff? Probably based on past behavior.