dnf.conf: comment fastestmirror and explain it in a comment

etc/dnf/dnf.conf

@@ -13,7 +13,10 @@ skip_if_unavailable=True
 deltarpm=False
 
 # Explicitly enable finding the fastest mirror
-fastestmirror=true
+# NOTE! This actually measures based on ping instead of available bandwitdh
+# and the Fedora infrastructure is already giving most suited mirrors, so
+# enabling this may lead to worse performance.
+#fastestmirror=true
 
 # default 3, max 20, often suggested 10. I am not sure high numbers bring
 # benefit, but double seems fine