added sentence about downside of https

docs/filesystem_layer/stratum1.md

@@ -36,6 +36,7 @@ However, EESSI provides a synchronisation server that can be used for setting up
     If this is the case, you can try switching to HTTPS by using `https://aws-eu-west-s1-sync.eessi.science` for synchronizing your Stratum 1.
     Even though there is no advantage for CVMFS itself in using HTTPS (it has built-in mechasnims for ensuring the integrity of the data),
     this will prevent the described issues, as the intrusion prevention system will not be able to inspect the encrypted data.
+    As HTTPS does introduce some overhead due to the encryption/decryption, it is still recommended to use HTTP as default.
 
 ### Manual configuration