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
When I run a mix of CF and Quad9 resolvers, Quad9 was consistently slower in response times when measured. But it didn’t make any perceivable difference in real usage.