improve documentation for protocol encryption

include/libtorrent/settings_pack.hpp

@@ -1468,6 +1468,17 @@ namespace libtorrent
 
 			// control the settings for incoming and outgoing connections
 			// respectively. see enc_policy enum for the available options.
+			// Keep in mind that protocol encryption degrades performance in
+			// several respects:
+			// 1. It prevents "zero copy" disk buffers being sent to peers, since
+			//    each peer needs to mutate the data (i.e. encrypt it) the data
+			//    must be copied per peer connection rather than sending the same
+			//    buffer to multiple peers.
+			// 2. The encryption itself requires more CPU than plain bittorrent
+			//    protocol. The highest cost is the Diffie Hellman exchange on
+			//    connection setup.
+			// 3. The encryption handshake adds several round-trips to the
+			// connection setup, and delays transferring data.
 			out_enc_policy,
 			in_enc_policy,