IPSec or L2TP/IPsec successfully established, but the USG admin GUI is unusable

HI,

I'm facing a very weird problem for the first time using several vendors firewall appliances. I've just deployed an USG60 behind a NAT model/router, opened port forwarding as required and configured IPSec VPN (Remote Access) to securely access the firewall from the internet using my MacBook and IPSecuritas client.

Testing the connection with my Mac connected to my mobile phone via tethering, I can see the tunnel correctly established (on the NAT modem/router I see proper logs, on the USG I see proper logs) and I can successfully reach the USG GUI login page pretty smoothly. But once I submit my credentials and I've been successfully authenticated, the navigation in the admin GUI gets incredibly slow, so much that it's actually useless.

If by converse I access via OpenVPN to a machine behind the USG (on its LAN) and from there I access the USG GUI, then everything works flawlessly as expected (just the same moment, the same machine, the same mobile phone, position and connection). Switching forth and back between IPSec on USG and OpenVPN on another server behind it produces consistent results: IPSec unusable, OpenVPN ok.

I can't figure what the problem might be. I've been modifying and relaxing the VPN configuration (e.g. weaker encryption) and even switched config to L2TP/IPsec using the native Mac OS client, getting the very same result nonetheless.