Webpages are really slow or even not loading. windows 10, chrome

dan niet, laat maar..

If you're using IPv6 on your network, please check out this article:

Also, as obfuscated some of the debug log, you may be interested in this method of securely sending us (the developers) some information:

Do you have a device that does not run on Win (maybe an Android smartphone) for further testing?

Does your IPv6 Internet connection work?
What does http://test-ipv6.com tell you?

Okay, this test shows that you don't have a working upstream IPv6 connection (everything that went out over IPv6 received a timeout error).

Do you have a firewall configured on your Raspberry? Can you post the output of

sudo iptables -L

You may have to install a rule that blocks port 443:

sudo iptables -A INPUT -p tcp --destination-port 443 -j REJECT

but I'm not sure if that is the cause of your problems.

Many problems with loading that takes forever is that HTTPS requests are not explicitly rejected by the Pi-hole and hence the browser is waiting forever, as nobody told him that his HTTPS request will never be answered.

This was not clear to me up to now. Reading your initial post it seems like you are having this problems since a few days but it wasn't clear that it was working fine at some time before.

I don't understand your question. The IPv6 test you ran earlier showed that you have no IPv6 connectivity at all.