The way RDM works is you hard-cast a spell with a short casting time, and then use your Dualcast buff to instantly cast a spell that would normally have a really long cast time.

For example at RDM 80, you would hard-cast Jolt II then Dualcast Veraero, hard-cast Verstone then Dualcast Verthunder, hard-cast Verfire then Dualcast Veraero, etc.

You consistently use a 2 second cast time spell to get a Dualcast buff, then you laugh at the 5 second cast time of the next spell.

Spell speed is pretty much useless on RDM because of this, everything has a 2 second cast time anyways. Then you throw in a built-in oGCD window for double-weaving abilities, and you have a very smooth class setup.