It’s recommended to replace at 79% or lower battery health. If performance is noticeably decreased be that point, consider having diagnostics performed by Apple Support.
It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.
If you’re in the US please call 1-800-MY-APPLE or outside the US, consult the support articles below.
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple
Explore Service Locations
Official Apple Support
Live Chat with an Apple Advisor in Messages.