Why are ad networks so slow with IPv6?

Verizon FIOS has been rolling out IPv6 across Northern Virginia. Hurrah!

Stuff with ads (which is almost everything on the modern Internet) now load much more slowly or timeout.

Changed IPv4/IPv6 connection preferences to use IPv4 first. Much improved user experience.

Is your experience the same if you run a browser-based ad blocker or something external like Pi-hole? I have Fios, but Verizon hasn’t rolled v6 out here that I can see. My v6 traffic runs over a tunnel to Hurricane Electric, and I haven’t noticed any unusually slow load times for the ads that make it through Pi-hole/uBlock/etc.

Thank you