Cloudflare DNS is a very fast, very reliable public DNS resolver. It is an enterprise-grade authoritative DNS service that offers great redundancy and advanced security with built-in unmetered and unlimited DDoS mitigation and one-click DNSSEC support. Cloudflare DNS is consistently used as a reverse proxy by many websites, and has a user-friendly interface and an available API.
It is only value-added to use Cloudflare DNS if your site has significant traffic or to provide additional protection against hackers. Cloudflare DNS offers limited security options, and it can potentially block genuine users to your site. As Cloudflare DNS is a middleman, if it goes down, so will your site.
Quad9 is a global public recursive DNS resolver and is run by a non-profit entity. Quad9 is a free security solution that is very efficient at protecting users from malware and phishing by using their unique curated block list. It offers real-time threat intelligence and protects users' privacy by not retaining or processing the IP addresses of its users. Consequently, it is GDPR-compliant. Quad9 can also help improve your system's overall performance. It is also available as an app for android users.
Quad9 can be very slow. Although it supports both IPv4 and IPv6 networks, it doesn’t support the DNS64 mechanism to translate IPv4 addresses for IPv6-exclusive networks. This can be problematic for some.
When deciding which solution may be the better fit, it really depends on what your usages are, preferences, and needs.
Regarding speed, Cloudflare DNS consistently performs at or near the top, beating out all other solutions. It is very fast, stable, and secure. So, if speed is a top priority, Cloudflare DNS is hands down the best choice.
Quad9 has some troubling speed issues depending on region/location. If blocking malware and phishing is paramount as well as keeping your DNS queries away from the for-profit sector then Quad9 is a great fit.