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When I request a website it starts to load within a second.
Many websites take about 10-20s to load, including www.google.com, www.facebook.com. When going to these a second time (having closed the page) they load nearly instantly. After 10 minutes or so the slow behaviour can return. Other websites are unaffected and load nearly instantly.
I have just setup Pi-hole on an old RaspberryPi 1 connected to me router which I changed the Primary DNS Observing the log in real time I have seen slow queries for both [AAAA] and [A] requests.
I have read various pages about this issue that suggest adding entries to iptables but these were all for <v4.0 . As far as I understand this had been resolved in newer versions and no longer necessary. Is this true or should I still try the suggestion on https://pi-hole.net/2018/02/02/why-some-pages-load-slow-when-using-pi-hole-and-how-to-fix-it/ ? If not, can you suggest anything else to try?