unfortunately, I identified this plugin as slowing down my backend - at least the plugins overview page.
I disabled all plugins one by one - Retina 2x is the last in the list :)
With all, the page was loading and loading awesome slow.
When deactivated Retina 2x, it just worked fine with all the others.
Also, I was wondering about the backend message for an plugin update available, but nothing was indicated in the plugin list. After deactivating Retina 2x also this update message vanished.