I'm well aware of how dumb home routers can be, but a general de-prioritizing of the transport protocol that DNS uses is both really dumb and pretty easy to detect. Are you aware of any router vendors that have actually shipped such a configuration? (Or for that matter, any consumer router that has shipped with any prioritization rules enabled out of the box?)
DNS isn't VoIP, but most DNS traffic is very much latency sensitive. When you click a link to a typical modern web page, you trigger multiple HTTP connections to load the page and its many resources hosted on dozens of domains. The DNS lookups are on the critical path for all of those requests.
Yes, but the HTTP requests themselves dominate the overall load time, even if DNS takes a bit longer. Plus there's caching to reduce somewhat the need for the DNS requests.
(Of course, I'm pulling all of this out of my nether regions without a lot of thought, and you may well be right, the sheer volume of such requests might lead to problems.)
It looks like on Windows a typical DNS retry timout is 1 second, with backoff if multiple retries are needed. If a DNS packet needed for loading a web page gets dropped, it's very likely to increase the overall page loading time. Re-ordering DNS to be delivered only when there's a lull in the large TCP packets would almost serialize the loading of resources from different domains.
DNS caching is a good thing, but it certainly doesn't eliminate this problem. Web browsing still produces a lot of DNS requests, and any cache upstream of the bottleneck (ie. any cache operated by your ISP rather than in your own router) doesn't help against loss during congestion.
Without active queue management, a single bulk upload can break DNS. When the bottleneck link (usually your cable/DSL modem) is saturated by the bulk upload, it will start dropping new packets that come in while the queue is full.
When a single packet worth of space frees up, the chances are that the bulk flow will instantly take it, such that nearly all outbound DNS packets get dropped, and DNS queries time out.
The solution is active queue management (CODEL is a good choice) at every bottleneck.
