Browser Developer Tools
Instead of adding longer values in your stylesheets.You can slow down transitions with Dev Tools to get a goog look at'em.
Uses Firefox Developer Edition (version 47). Also possible with Chrome.
Firefox: Animation tab (where Rules and Computed are)
You can set the entire animation rate.
Chrome: Bring up the console, then Animation tab. You can adjust the speed by percentage. If you don't have the Animation tab, then click the three vertical dots left of "console" tab.