I tried Quad9 at our business for a while, about a year. I ended up moving to something else due to latency. I'm not sure if it was a routing issue or what but there were a lot of timeouts and slow responses. I have not had these issues with other providers.

I'd used NextDNS for a while but the service kept degrading with more timeouts regardless if you were on free/paid. Haven't had any issues with Quad9