Comcast isn't doing anything to your DNS. They're the largest ISP in the country, there'd be a huge uproar if they were doing something like that. There are plenty of experts who are subscribers who'd be able to figure out exactly what's going on.
https://datatracker.ietf.org/doc/html/draft-livingood-dns-re...
Their own words
It's less common, I think, because more people know how to check their speed than change their DNS.
Or, somehow more cynically, the ISP makes money from selling the data collected from DNS, so punishes people who use a different DNS provider. (DNS is plaintext-by-default, so I don't quite see how this would work, but it's possible.)
Or perhaps the system uses DNS lookups as a proxy for “is a human browsing the web”; if there aren't enough, it's clearly some kind of automated computer program that doesn't deserve internet access.
I know the best answer for a Comcast DNS server in New York is the server I physically installed in a New York Comcast rack, but when a public DNS server asks me from Paris, maybe I suggest a London server, 'cos that's pretty close to Paris, shame that New York isn't.
EDNS Client Subnet is a feature that lets a DNS server say OK, I'm asking on behalf of somebody from 10.20.30/24 and so my system can do the same trick with ECS. But doing this unwinds most of the privacy benefit of using a public service, so several famous public DNS servers explicitly do not use ECS.
Obviously the cheap bulk host used for some Single Serving site like "Is pizza rat mayor of New York yet?" isn't affected, that is only one server and it is wherever it is, but somebody like Netflix absolutely is affected by this because they have their machines close to the customers to deliver better performance and if they don't know where the customer is that inteferes.
QUIC has an optional feature called Connection Migration to help improve this, the remote server is like "Um, now that you're connected to www.example.com here in Glasgow, Scotland, I notice your IP address is from Tokyo, Japan, and this is just a suggestion, but maybe talk to my identical twin also named www.example.com in Tokyo, Japan for better performance? Here is the IP address to try"
https://arstechnica.com/tech-policy/2009/08/comcasts-dns-red...
That's aside from all the political shenanigans they pull, trying to kill municipal broadband and net neutrality. There's scarcely a more evil ISP.
I switched away from Comcast as quickly as I could, and never had trouble with either smaller cable companies or fiber providers.