Hi
pfSense caches all DNS requests in DNS resolver mode, so will a PC and most other devices after the first request so there simply isn't that much DNS traffic on a typical network and very few packets.
If you go into pfSense, select pfTop in Diagnostics, and under the filter expression type 'port 53' to view all DNS traffic going in and out, you will see how much traffic there is related to DNS, on my network with lots of devices there can be lots of requests but they are just a few packets and over and done with in a split second before the firewall times them out after 30 seconds or so, and fewer still tend to go out to the wider Internet, with a lot being resolved from the pfSense DNS cache. Compared to all the other traffic going in and out, DNS is just a very tiny fraction of data, and to the router, VM modem and their network, a DNS request is no different to anything else.
So I'm not sure why the forwarder mode would make any difference, however there is a bug with pfSense when the DNS Resolver is enabled in that it keeps reloading if the option 'Register DHCP static mappings in the DNS Resolver' is checked (under Servers - DNS Resolver) this can cause some down time when DNS isn't being resolved and makes pfSense a bit busy for a bit especially where you have other packages installed that also restart because that.
Your chart looks typical really for Virgin Media really, was your neighbours chart also from Virgin Media?