Timestamp resolution in pihole.log is too coarse to allow gauging DNS response times.
Pi-hole's current Query Log provides response times under its Reply column.
If you don't see a flood of values well in and above the hundreds of ms there while you experienced a slow loading page, we can rule out slow host name resolution itself as a cause for your delays.
But the portion of the log you provided shows that you are not only using Google as DNS, but also others like 2620:0:ccc::2 or OpenDNS (208.67.220.220) . At least that last one is providing it's own filtering when resolving hosts, which may or may not happen to disagree with facebook.
Try getting rid of additional DNS servers and see if you receive the same slow loading results.