I have continued and foundĀ the connection drawing code also makes “hidden” checks, and it is this code that is responsible for the slowdown (Editor.Node.cs ln 104).
As a workaround I return false right at the start of the get method (my client and I are just getting started so are unlikely to hide things for a while I think!), and the canvas runs fine again.
(PS. I didn’t mention before, I am using the Dialogue Tree)