I am confirming I am experiencing the same behavior on non-windows non-firefox devices, such as when browsing to reddit.com on my iPhone running iOS 16.2.
Reddit simply does not load. When I disable wifi and use 5G (thereby bypassing my DNS) or more importantly, when I use wifi and choose manual DNS, removing my pi-hole addresses (both ipv4 and ipv6 address) and using my router DNS forwarder (1.1.1.1), Reddit loads instantly.
Why is this happening?
Nowhere on my pi-hole admin console is it being blocked. It's simply not making the request.
In your original post and this one there's no mention of how you are connected to your network. Could this be worthy of investigation?
I've had the same symptoms in the past when I was using powerline adapters to connect my main computer to the router in another room. They would report they were linked but had these random long periods of 30+ seconds where web pages, ssh, etc were waiting to load. Clearly it was not an ideal layer over which to run networking. I got the impression that under the hood these adapters were having to constantly and invisibly renegotiate their connections below the networking.
I've also had similar symptoms from a router that worked well for a couple of years and then started "giving problems" (some TP-Link model, can't remember which). This led to the same kind of long pauses. I suspect the wifi was being driven to the point it was starting to fail, which results in another messy link that has to be constantly renegotiated below the networking.
Your ISP may also be implicated. In the UK we have contended broadband and some providers under-provision and over-sell, leading to slow downs and timeouts at peak times.
The fact that you're seeing this across devices and applications on your network, and not seeing it when on 5G, implies this is not a problem with Pi-hole or even necessarily with your configurations. The Pi-hole and DNS delays appear to be just more victims of whatever is really going on.
If I was troubleshooting this I would switch everything off apart from the router and your computer – no smart devices, no TVs, no Pi-hole – and just use the computer and router as normal. Are you still seeing the problem? If so then the connection angle seems to have traction.
If it's working fine then now introduce just the Pi-hole and manually set the computer to use it for DNS. How is that performing? Hopefully okay. Then adjust the Pi-hole/router so it's the way you are using it now. Is it still okay?
If so then I would bring everything back online and see if the problem resumes. That at least shows it's contained outside the known good computer / router / Pi-hole combo, and you can take them all offline again and start to investigate them. For example, perhaps one of them is producing malformed DHCP ARP requests and the router network stack keeps falling over and re-loading, which you experience as these delays.
Also try replacing www.reddit.com with old.reddit.com (which loads the old interface) to see if it is somehow related to the domain or any of the myriad of scripts it runs.