This seems not only related to migration (gtk2/Gtk3, Introspection)?

Maybe I should also play with profile.

Before, any idea of causes?

Since I only used gtk3-based gramps in a VM, I hoped that the slowdown I perceived visually was due to the VM overhead, but, alas, it looks like I was wrong.
Profile is your friend, but I reckon it's mostly due to the gtk3/gi transition.

I hope this is not a release blocker, this slowdown seems to have been there for ages...