Probably the responsiveness of things. Firefox is very sensitive to DNS roundtrip time during daily use. A faster response time provides much better experience with it.
I guess that ~25% of "Firefox is slow" myth is coming from slow DNS response, if not higher.
I honestly have no idea. I observe it all the time, and note repeatedly everywhere when the discussion comes up, but never had the time to dig into the code and see how that all works.
That seems pretty reasonable to me. The vast majority of innovations that have consistently raised everyone's standard of living over the last centuries has also come from that 10%.
A regular dns like quad9 + ublock origin on Firefox has been a consistently great experience for me.