Why do some sites take forever to load when using Pi-hole? (for versions < v4.0)

[DL6ER]

Note that within the FTLDNS beta testing branches, there is a new option available that allows NXDOMAIN blocking. This should solve all these problems if you're willing to try it out.

Slow PiHole

Have a look at the README of the FTLDNS branch. Search for a setting called BLOCKINGMODE. Setting this to NXDOMAIN should, at least, resolve the firewall issues. Note that it'll disable the blocking page as a side effect.