resolv.conf and sporadic slow host resolving (DNS issue) - Ubuntu 20.04

Question

since last autumn I'm struggeling to find what's wrong with my network configuration on Ubuntustudio 20.04 which is basically a Ubuntu 20.04.

I installed from scratch last autumn with the official Ubuntustudio 20.04 iso.

The strange thing is that my resolv.conf links to ../run/resolvconf/resolv.conf

    /etc/resolv.conf
lrwxrwxrwx 1 root root 29 Sep 27 22:10 /etc/resolv.conf -> ../run/resolvconf/resolv.conf

However, in the point release version 20.04.1 it obviously points to ../run/systemd/resolve/stub-resolv.conf.

The content in ../run/resolvconf/resolv.conf is

nameserver 127.0.0.53
search fritz.box

I'm having another resolv.conf file in /run/systemd/resolve with exactly the same content. The time stamps of both files are different though: The file in ../run/resolvconf has the time stamp of the time when I switched on my machine. The file in /run/systemd/resolve/ has a time stamp which is 30 min old.

In the folder /run/systemd/resolve/ there's also a stub-resolv.conf file with content:

nameserver 127.0.0.53
options edns0 trust-ad
search fritz.box

Does this all make sense and can somebody clearify this? Could this be the cause of my DNS issues? Very sporadically the DNS lookup in Firefox takes 5 s or more. This happens about 20 time per day or more, but not always. When I tell Firefox to use DNS over HTTPS all is fine.

When I use dig in the terminal this also works most of the time, but sometimes there's a delay of several setting until I get any output. The DNS lookup times in this case are however also short on the order of 20 ms.

For instance this took ~10 s until I got the first output

traceroute facebook.com
traceroute to facebook.com (69.171.250.35), 30 hops max, 60 byte packets
 1  fritz.box (192.168.178.1)  9.270 ms  9.211 ms  9.272 ms
 2  loopback1.0002.acln.01.off.de.net.telefonica.de (62.52.200.219)  47.276 ms  47.403 ms  47.400 ms
 3  bundle-ether12.0001.dbrx.01.off.de.net.telefonica.de (62.53.12.0)  24.104 ms  24.351 ms  27.042 ms
 4  ae3-0.0001.corx.01.off.de.net.telefonica.de (62.53.28.154)  31.595 ms  36.469 ms  36.011 ms
 5  ae10-0.0001.corx.06.ham.de.net.telefonica.de (62.53.0.48)  33.152 ms ae4-0.0002.corx.02.ber.de.net.telefonica.de (62.53.0.72)  36.903 ms ae10-0.0001.corx.06.ham.de.net.telefonica.de (62.53.0.48)  35.555 ms
 6  bundle-ether1.0006.dbrx.01.ham.de.net.telefonica.de (62.53.14.27)  35.957 ms ae1-0.0002.dbrx.01.dus.de.net.telefonica.de (62.53.16.36)  24.730 ms bundle-ether2.0005.dbrx.01.ham.de.net.telefonica.de (62.53.14.231)  26.659 ms
 7  ae0-0.0002.prrx.01.ham.de.net.telefonica.de (62.53.6.109)  26.043 ms ae1-0.0002.prrx.01.ham.de.net.telefonica.de (62.53.6.207)  26.547 ms  27.471 ms
 8  ae0.pr02.ham3.tfbnw.net (157.240.72.132)  25.267 ms  25.832 ms  26.783 ms
 9  po102.psw04.ham3.tfbnw.net (74.119.78.17)  26.633 ms po102.psw03.ham3.tfbnw.net (74.119.78.5)  26.436 ms po102.psw01.dus1.tfbnw.net (157.240.35.105)  23.561 ms
10  157.240.38.185 (157.240.38.185)  23.765 ms 157.240.38.73 (157.240.38.73)  25.523 ms 173.252.67.59 (173.252.67.59)  25.862 ms
11  edge-star-mini-shv-01-any2.facebook.com (69.171.250.35)  24.973 ms  26.343 ms  24.574 ms
systemd-resolve --status
Global
       LLMNR setting: no

MulticastDNS setting: no

  DNSOverTLS setting: no

      DNSSEC setting: no

    DNSSEC supported: no

          DNS Domain: fritz.box

          DNSSEC NTA: 10.in-addr.arpa

                      16.172.in-addr.arpa 
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa 
                      18.172.in-addr.arpa 
                      19.172.in-addr.arpa 
                      20.172.in-addr.arpa 
                      21.172.in-addr.arpa 
                      22.172.in-addr.arpa 
                      23.172.in-addr.arpa 
                      24.172.in-addr.arpa 
                      25.172.in-addr.arpa 
                      26.172.in-addr.arpa 
                      27.172.in-addr.arpa 
                      28.172.in-addr.arpa 
                      29.172.in-addr.arpa 
                      30.172.in-addr.arpa 
                      31.172.in-addr.arpa 
                      corp

                      d.f.ip6.arpa

                      home

                      internal

                      intranet

                      lan

                      local

                      private

                      test
Link 2 (enp2s0)
      Current Scopes: DNS

DefaultRoute setting: yes

       LLMNR setting: yes

MulticastDNS setting: no

  DNSOverTLS setting: no

      DNSSEC setting: no

    DNSSEC supported: no

  Current DNS Server: 192.168.178.1
         DNS Servers: 192.168.178.1
          DNS Domain: ~.

                      fritz.box

One more info maybe: I was using a WLAN stick for roughly one week and didn't have such issues. Long DNS times occured only with LAN:

Network:   Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet driver: r8169 
           IF: enp2s0 state: up speed: 1000 Mbps duplex: full mac: d8:cb:8a:e8:e9:99