1.5s is not an exaggeration. RDM only has a cast time of 2s which with a lowly 1000 spell speed is roughly 1.94s and if you need to move you can do it as early as 1.5s or so and not lose the cast. Most of the you don't need to so it doesn't matter, but when you need to move then you only need to cast for 1.5s.