Build or update inarray GIST index very very slow.
Yes, it is. I think there is room for improvement for the algorithm for picking the pivot.
But in documentation "As a rule of thumb, a GIN index is faster to search than a GiST index, but slower to build or update; so GIN is better suited for static data and GiST for often-updated data."
Rules of thumb are not always true, that is what makes them rules of thumb.
Also, note that that sentence was removed from the documentation in 9.5 onward (commit 8e1ad1b37c7afdf99). I can't find the mailing-list thread discussing it, but I think that that advice was deemed too vague to be useful.