I’ve experienced the same problem when I setup my Pioneer FreedomBox — page refreshing could take anything between 1 or more minutes (or fail altogether). After searching this forum I noticed that some people in the past reported slow navigation due to Ethernet speed types (Giga- vs Mega-bit connections) and I realized that the cause might be that I was connecting to network via a powerline adaptor (ethernet via electric line).
I purchased a Lan switchbox and a long cable so I could replace the powerline adaptor via a direct ethernet cable connection to my router and everything worked fine, since then Plinth pages load times are reasonable (like an Apache server running on localhost).
I’m not sure why the powerline adapter was rendering Plinth so slow — in terms of Internet connectivity, all other devices using it where running very fast, only FreedomBox was suffering slow connection — so maybe some of the tech guys might look into it.
Specifically, my powerline adapter was a NetGear PL1000v2 (Powerline 1000):