Dihedral is for adding stability to a design. Not something that is usually desireable for a fun fly type. It also would cause a roll coupling with the rudder resulting in sloppy wingovers and other rudder type stuff. Not to mention that when you're upside down all the tendencies are suddenly reversed. Also dihedral by itself does not slow or speed up a model. It's just that dihedral is usually found on less demanding to fly model designs so perhaps that's why you asked about the speed.

Just make it flat. There's nothing to be gained by adding dihedral to a 3D fun fly model.