This is very similar to this report from 2008. I switch between classes, look at the Structure panel, get confused what is going on, and what is going on is that the Structure tool still shows the members from the class that I had opened before.
I have this tool window always open and this disturbs my workflow. Sometimes I'm not even interested in the content of the editor window, I look right at the Structure window but when the delay is 1 or 2 seconds it becomes a problem. I get this lag even when switching between simple classes or interfaces.
I open the same project in Netbeans and there is no lag at all. Very occasionally, normally when running some other tasks in the background, Netbeans will show me a clear indication, an hourglass, that it is not ready to show me the members. Idea instead shows me a stale view, not good.