As you stated you are testing TCP and UDP, so I assume you are using OpenVPN and that NordVPN is probably using AES-256 encryption which is very secure and very slow to calculate on the CPU in a Mango. You can check to see if they support AES-128, and how to enable it, which may speed things up by up to a factor of 2. The other choice is to see if they support Wireguard VPN, which runs much faster on these small routers, as they use a different encryption method that is quicker to calculate with the CPU used in the Mango.
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