Docs: mention performance reasons for reusing RSAPrivateKey when enco…

…ding (#734)

* Mention reusing RSAPrivateKey for performance reasons

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docs/usage.rst

@@ -45,6 +45,9 @@ If your private key needs a passphrase, you need to pass in a ``PrivateKey`` obj
     )
     encoded = jwt.encode({"some": "payload"}, private_key, algorithm="RS256")
 
+If you are repeatedly encoding with the same private key, reusing the same
+``RSAPrivateKey`` also has performance benefits because it avoids the
+CPU-intensive ``RSA_check_key`` primality test.
 
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