I can concur there are some serious performance issues with adding Graphics Elements in 2.5 when other tabs are open. We have a slightly different situation but it's essentially the same result.
Our add-in does some map frame cloning and adding quite a few graphic elements on a number of layouts.
If we have ANY layout open (not just the ones we are working on) then our code finishes (not quickly but it does finish) but Pro then goes into a unresponsive state for up to 4 hours (around 20 layouts). If we close all layouts programatically before we start then it only hangs for around 15 minutes.
It think it also may be related to your post on 11 May with bulk adding of GraphicElements and Uma's bulk creation overloads where i would imagine they are disconnecting UI updates for the duration of the bulk addition.
What would be more useful would be a way to programatically turn UI updates off for the duration of a Task and only have UI update at the end.