Rear Difusser for C4

[Nelson84]

I was looking at other cars and some have rear difussers on them. Is it cause the C4 is to slow?

 

[vintageracer]

Probably the aerodynamics of a C4 are so perfect that they do not need one o' them afterthought diffusers.
Purity of design!

 

[NoSkillz]

I was looking at other cars and some have rear difussers on them. Is it cause the C4 is to slow?

Highly doubt the determining factor was it being to slow. Are you comparing this to cars of matching generation? Post up some pics maybe?
Most diffusers seen on cars today aren't very piratical and are really just aesthetic.