I have a new network setup and am I'm trying to get my new pi-hole working. My equipment is: Ubiquiti USG (router/firewall), 8-port managed switch, and AP AC Lite. Pi-hole is on subnet 192.168.1.x along with the USG.
I don't believe that there is an issue with the communication on the network and the pi-hole.
When I connect to websites the typical behavior is that non-blocked items are loading, then blocked items will sit and try to connect until they timeout. Below is the Chrome inspector from pi-hole's ad page as an example.
I did change the iptables per previous posts on the forum and did not see any change in behavior. I have ipv6 disabled on my modem and by default the unifi equipment does have ipv6.
If you click on one of the requests in the dev tools (such as in your screenshot), it opens up with the request information, including a tab called Timing.
If you try to connect to the web interface over https, does it time out?