NextDNS sends EDNS client subnet (ECS). If challenged on privacy grounds they can claim it is for performance but a primary benefit of ECS, whether intentional or not, is to serve online advertising interests.^1

1. Dishonest people might try to debate intentionality. But forseeability is indisputable. The privacy issues created by ECS were known when it was introduced by Google. If ECS is truly for performance _that benefits the user_ then it stands to reason that it should the _user's_ choice whether to send it. That is, ECS should be optional. This is not merely a personal opinion. It was a consensus. See: https://yacin.nadji.us/docs/pubs/dimva16_ecs.pdf
AFAIK, NextDNS, like Google and OpenDNS, will not allow any user to disable sending ECS.

For example, Cloudflare when it launched 1.1.1.1 decided not to send EDNS subnet and they have claimed this is based on privacy grounds.

Whether anyone cares about privacy is their business, not mine. And whether anyone believes ECS improves peformance for them is for them to decide, not me.^2 Here I am just presenting some facts for consideration. Anyone is free to disregard these facts.

2. When considering "performance" we might differentiate between performance in requesting the resource the user is trying to access versus performance of ad servers or tracking servers. Needless to say, ads are not the resource the user is trying to access. And tracking is not even a resource. The speed of ads and tracking are obviously very important to Google, the company behind ECS. When we see a campaign for a "faster internet" from so-called "tech" companies such as Gooogle and Facebook we should keep in mind that "the internet" as envisioned by these middlemen is an internet full of advertising and tracking. As such, "faster internet" does not necessarily mean better speeds when downloading a resource. Ads and tracking are the not resources that users are intentionally requesting. They only serve to add delay and impede the user's retrieval of a desired resource. Hence the need for "ad blocking".

Personally, I do not use third party DNS services, i.e., shared DNS caches operated by third parties. Historically these shared caches are the source of various problems. There are plenty of alternatives available today what with the enormous advances in network speeds and local storage that have occurred since the days when shared DNS caches were a necessity. For example, all the DNS data I use is stored locally and served from loopback addresses, either in the memory of a forward proxy or from authoritative DNS servers. Requests never leave the computer. (NB. PiHoles send requests to upstream third party DNS providers by default. Unless the parent commenter changed the PiHole's i.e., dnsmasq's, configuration to use a local DNS server serving locally stored DNS data then requests would by default be sent to the internet. In the case the configuration is changed to point to a local DNS server serving local DNS data and the user is satisfied with DNS-based blocking, like what NextDNS provides, then the utility of a PiHole would be questionable. Just omit DNS data for ad/tracking servers. I have been doing this for decades; I began using DNS for "blocking" before "adblockers" or PiHole existed.)