I'm experiencing similar issues with Instagram loading times. Content just doesn't seem to load very fast/at all. The moment I disconnect from WIFI and to 4G it loads completely fine.
This is the same across multiple devices accessing different Instagram accounts.
My actual internet speeds (and all other websites) load completely fine. (I consistently get 100mb speeds)
A friend who also lives in the area, with a Superhub 2, does not experience the same issues leading me to think it could be my Superhub 3.
I have tried disabling Smart Wifi and also the Websafe feature but neither have worked. I will look into disabling the firewalls but surely that cannot be a long term solution? I would rather not compromise my devices security.