Connection (web interface and syncing) slow when using ddns, but fast with local address

Ok, so i’ve been using nextcloud for a while now and always complained because it works too slow for me.
Now I updated to NC12 and tried syncronization on a new machine and tested connecting using the static IP the nextcloud-server has. I tried syncing a 1GB folder and navigating through the webpage.
The result: fast web interface, (relatively) fast syncronization (~7MBps).

I then repeated the process (web browsing and syncing) using a ddns (two, actually, one from and the other from
The result: slow login webpage, slow web.browsing, slow syncronization (between 7~26KBps).

Now I don’t know where to troubleshoot.
I thought it could be the port forwarding in the router, but since I
do load the webpage (although slowly) i’m not convinced.
I thought it could be the webserver on the nextcloud machine… I don’t get why.

How to troubleshoot?

I’m running NC12 stable on Lubuntu 16.04 with PHP7, Nginx and Letsencypt.

Thanks for reading!