Added comment on CompCert vs. gcc performance

paper/answer-Usenix.md

@@ -24,8 +24,16 @@ REVIEW 1:
   implementation).
 
 * Does compiling TweetNaCl with CompCert rather than gcc impact the performance
-  beyond what is acceptable? If so, what trust do you consider this proof effort
-  to bring to a gcc compiled implementation?
+  beyond what is acceptable? 
+
+  For the X25519 implementation in TweetNaCl, CompCert generates code that is
+  about 6x slower than code generated with gcc. While this sounds like a lot, it
+  may not be too much of an issue for projects that use TweetNaCl, because they
+  chose for a library that prioritizes simplicity over performance in the first
+  place. A more serious issue however can be the non-free CompCert license.
+  
+* If so, what trust do you consider this proof effort to bring to a gcc compiled
+  implementation?
 
   We are not aware of any bugs that were introduced into ECC software by bugs in
   gcc; so from a practical, crypto-engineering point of view we rule out all