Pppoe-passthrough issues

The provider that i use, supports pppoe-passthrough. So i attached my Openwrt router and i am getting an IPv4/IPv6 address, not CGnat. My port forwards work fine and all of the services that i use.
There are a few issues with delay though. slight delay to open the web pages, in overall speed, increased time in ping to www.google.com, viber attachments, email attachments.
I tried to change the MTU size on the pppoe interface of Openwrt from the default to different values as low as 1300, without any changes.

Any suggestions, ideas, please?

If you use Linux on a machine, can use dnseval to check latency on several DNS servers.

Make sure your DNS server is in the list.

sudo apt install dnsdiag

cat <<EOF > /tmp/dns.txt
1.0.0.1
1.1.1.1
4.2.2.1
4.2.2.2
4.2.2.3
4.2.2.4
8.8.4.4
8.8.8.8
208.67.220.220 
208.67.222.222
209.244.0.3
EOF

dnseval -t A -f /tmp/dns.txt -c40 microsoft.com

server             avg(ms)     min(ms)     max(ms)     stddev(ms)  lost(%)  ttl        flags                  response
----------------------------------------------------------------------------------------------------------------------
1.0.0.1            7.439       6.710       8.495       0.406       %0       923        QR -- -- RD RA -- --   NOERROR             
1.1.1.1            8.050       6.866       14.409      1.533       %0       1022       QR -- -- RD RA -- --   NOERROR             
4.2.2.1            6.901       6.289       10.272      0.701       %0       3169       QR -- -- RD RA -- --   NOERROR             
4.2.2.2            6.924       6.200       8.923       0.706       %0       3169       QR -- -- RD RA -- --   NOERROR             
4.2.2.3            7.654       6.341       25.971      3.196       %0       155        QR -- -- RD RA -- --   NOERROR             
4.2.2.4            7.000       6.245       13.368      1.153       %0       1102       QR -- -- RD RA -- --   NOERROR             
8.8.4.4            7.578       6.472       14.265      1.501       %0       1867       QR -- -- RD RA -- --   NOERROR             
8.8.8.8            7.397       6.478       14.281      1.328       %0       1527       QR -- -- RD RA -- --   NOERROR             
208.67.220.220     7.648       7.063       8.869       0.475       %0       3052       QR -- -- RD RA -- --   NOERROR             
208.67.222.222     7.624       7.010       11.009      0.725       %0       2116       QR -- -- RD RA -- --   NOERROR             
209.244.0.3        7.102       6.329       13.489      1.136       %0       1100       QR -- -- RD RA -- --   NOERROR

problem is present when i am NOT using the ISP's DNS
εικόνα

Good case: If i use the ISP's DNS

Bad case: If i use any of the following DNS servers 1.1.1.1, 8.8.8.8, 8.8.4.4, 1.0.0.1, 9.9.9.9

Here are the DNS diagnostics from my LAN:

So how do i bypass the ISP's DNS without having the issue i do mention above?