Have you measured the hops and packet loss to these resolvers? Install mtr if you don’t have it already and do a mtr -rwc 5 1.1.1.1 then mtr -rwc 5 8.8.8.8 etc.
Then install dnsutils and do a time dig @1.1.1.1 dietpi.com then time dig @8.8.8.8 dietpi.com
It is generally better to have multiple nameserver entries in the resolv.conf for high availability in case of failure.