After a transaction is completed (Some actions is performed), then yumex has to reload the yum backend to get a fresh picture of how the world look like now, that can't be avoided.
But the long time it can take is not caused by it take to reload yum information, it is the time GTK takes to add many element to the TreeView, it is a real pain to add 10000+ packages to the view.
It is much faster to just stand on the update page and search for package, then to select the available page and wait for the packages to load into the view :)
Tim