Update README.md

README.md

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 This Python 3 code implements Galois field (finite field) arithmetic in pure Python,
 leveraging Python's support for very long integers.
 
+Because it's in pure Python, it's relatively slow --- don't use it for performance-critical
+applications.
+Instead, use it to _verify_ other implementations, or to understand how Galois field calculations work.
+
 ## Authors
 
 - Ethan L. Miller (`coding@ethanmiller.com`)